U.S. patent application number 12/727438 was filed with the patent office on 2010-09-30 for electrode for lithium ion secondary battery and lithium ion secondary battery.
This patent application is currently assigned to TDK Corporation. Invention is credited to Kazutoshi EMOTO, Kiyonori HINOKI, Haruka NISHIMURA, Masahiro SAEGUSA.
Application Number | 20100248026 12/727438 |
Document ID | / |
Family ID | 42772233 |
Filed Date | 2010-09-30 |
United States Patent
Application |
20100248026 |
Kind Code |
A1 |
HINOKI; Kiyonori ; et
al. |
September 30, 2010 |
ELECTRODE FOR LITHIUM ION SECONDARY BATTERY AND LITHIUM ION
SECONDARY BATTERY
Abstract
An electrode for a lithium ion secondary battery having a
collector, an active-material layer formed on the collector and a
protecting layer formed on the active-material layer, in which the
protecting layer contains an organic particle formed of poly(methyl
methacrylate) having a crosslinked structure, and the organic
particle has an average particle size (D50) of 0.5 to 4.0
.mu.m.
Inventors: |
HINOKI; Kiyonori; (Tokyo,
JP) ; EMOTO; Kazutoshi; (Tokyo, JP) ;
NISHIMURA; Haruka; (Tokyo, JP) ; SAEGUSA;
Masahiro; (Tokyo, JP) |
Correspondence
Address: |
OLIFF & BERRIDGE, PLC
P.O. BOX 320850
ALEXANDRIA
VA
22320-4850
US
|
Assignee: |
TDK Corporation
Tokyo
JP
|
Family ID: |
42772233 |
Appl. No.: |
12/727438 |
Filed: |
March 19, 2010 |
Current U.S.
Class: |
429/209 |
Current CPC
Class: |
H01M 4/62 20130101; H01M
10/0525 20130101; H01M 4/366 20130101; H01M 4/133 20130101; H01M
4/13 20130101; Y02E 60/10 20130101; H01M 50/46 20210101; H01M
10/4235 20130101 |
Class at
Publication: |
429/209 |
International
Class: |
H01M 4/02 20060101
H01M004/02 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 25, 2009 |
JP |
2009-074171 |
Mar 25, 2009 |
JP |
2009-074214 |
Mar 25, 2009 |
JP |
2009-074225 |
Mar 25, 2009 |
JP |
2009-074746 |
Claims
1. An electrode for a lithium ion secondary battery comprising a
collector, an active-material layer formed on the collector and a
protecting layer formed on the active-material layer, wherein the
protecting layer contains an organic particle formed of poly(methyl
methacrylate) having a crosslinked structure, and the organic
particle has an average particle size (D50) of 0.5 to 4.0
.mu.m.
2. The electrode for a lithium ion secondary battery according to
claim 1, wherein the organic particle has a shape satisfying
conditions expressed by the following expression (1).
1.00.ltoreq.(major-axis length/minor-axis length).ltoreq.1.30
(1).
3. A lithium ion secondary battery comprising a positive electrode
and a negative electrode, wherein at least one of the positive
electrode and the negative electrode is an electrode having a
collector, an active-material layer formed on the collector and a
protecting layer formed on the active-material layer, the
protecting layer contains an organic particle formed of poly(methyl
methacrylate) having a crosslinked structure, and the organic
particle has an average particle size (D50) of 0.5 to 4.0
.mu.m.
4. The lithium ion secondary battery according to claim 3, wherein
the organic particle has a shape satisfying conditions specified by
the following expression (1): 1.00.ltoreq.(major-axis
length/minor-axis length).ltoreq.1.30 (1).
5. The lithium ion secondary battery according to claim 3, wherein
at least the negative electrode is an electrode having the
collector, the active-material layer and the protecting layer.
6. An electrode for a lithium ion secondary battery comprising a
collector, an active-material layer formed on the collector and a
protecting layer formed on the active-material layer, wherein the
protecting layer contains an organic particle and an inorganic
particle, the organic particle has a melting temperature of 100 to
200.degree. C., the organic particle and the inorganic particle
each have an average particle size (D50) of 0.10 to 4.0 .mu.m, and
a ratio of a content of the organic particle relative to a content
of the inorganic particle in the protecting layer is 1:1 to 1:4, in
terms of mass.
7. The electrode for a lithium ion secondary battery according to
claim 6, wherein the organic particle is a particle formed of
polyethylene.
8. A lithium ion secondary battery comprising a positive electrode
and a negative electrode, wherein at least one of the positive
electrode and the negative electrode is an electrode having a
collector, an active-material layer formed on the collector and a
protecting layer formed on the active-material layer, the
protecting layer contains an organic particle and an inorganic
particle, the organic particle has a melting temperature of 100 to
200.degree. C., the organic particle and the inorganic particle
each have an average particle size (D50) of 0.10 to 4.0 .mu.m, and
a ratio of a content of the organic particle relative to content of
the inorganic particle in the protecting layer is 1:1 to 1:4, in
terms of mass.
9. The lithium ion secondary battery according to claim 8, wherein
the organic particle is a particle formed of polyethylene.
10. The lithium ion secondary battery according to claim 8, wherein
at least the negative electrode is an electrode comprising the
collector, the active-material layer and the protecting layer.
11. An electrode for a lithium ion secondary battery comprising a
collector, an active-material layer formed on the collector and a
protecting layer formed on the active-material layer, wherein the
protecting layer contains a low-melting point material having
melting temperature of 100 to 200.degree. C., and a high-melting
point material having melting temperature of 300.degree. C. or
more.
12. The electrode for a lithium ion secondary battery according to
claim 11, wherein the low-melting point organic particle is a
particle formed of at least one material selected from the group
consisting of polyethylene, polypropylene and poly(methyl
methacrylate).
13. The electrode for a lithium ion secondary battery according to
claim 11, wherein the high-melting point organic particle is a
particle formed of at least one material selected from the group
consisting of polyimide and polytetrafluoroethylene.
14. The electrode for a lithium ion secondary battery according to
claim 11, wherein the ratio of a content of the low-melting point
organic particle relative to a content of the high-melting point
organic particle in the protecting layer is 1:1 to 1:4, in terms of
mass.
15. A lithium ion secondary battery having a positive electrode and
a negative electrode, wherein at least one of the positive
electrode and the negative electrode is an electrode having a
collector, an active-material layer formed on the collector and a
protecting layer formed on the active-material layer, and the
protecting layer contains a low-melting point material having
melting temperature of 100 to 200.degree. C. and a high-melting
point material having melting temperature of 300.degree. C. or
more.
16. The lithium ion secondary battery according to claim 15,
wherein the low-melting point organic particle is a particle formed
of at least one material selected from the group consisting of
polyethylene, polypropylene and poly(methyl methacrylate).
17 The lithium ion secondary battery according to claim 15, wherein
the high-melting point organic particle is a particle formed of at
least one material selected from the group consisting of polyimide
and polytetrafluoroethylene.
18. The lithium ion secondary battery according to claim 15,
wherein the ratio of a content of the low-melting point organic
particle relative to content of the high-melting point organic
particle in the protecting layer is 1:1 to 1:4, in terms of
mass.
19. The lithium ion secondary battery according to claim 15,
wherein at least the negative electrode is an electrode having the
collector, the active-material layer and the protecting layer.
20. A lithium ion secondary battery comprising a pair of electrodes
facing each other, and a separator interposed between the
electrodes, wherein at least one of the electrodes has a protecting
layer, an active-material containing layer and a collector
sequentially from the separator, the protecting layer contains a
silicone resin particle having at least one of structural units
represented by RSiO.sub.1.5 and R.sub.2SiO (in the formula, R
represents an alkyl group having 1 to 6 carbon atoms or a phenyl
group).
21. The lithium ion secondary battery according to claim 20,
wherein the silicone resin particle has an average particle size of
0.3 to 6.0 .mu.m.
22. The lithium ion secondary battery according to claim 20,
wherein the protecting layer has a thickness of 0.3 to 6.0
.mu.m.
23. The lithium ion secondary battery according to claim 20,
wherein an aspect ratio, which is a ratio of a major-axis length
relative to a minor-axis length of the silicone resin particle is
1.00 to 1.50.
24. The lithium ion secondary battery according to claim 20,
wherein the silicone resin particle is a polymethylsilsesquioxane
particle.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to an electrode for a lithium
ion secondary battery and a lithium ion secondary battery.
[0003] 2. Related Background Art
[0004] As a safety measure for a lithium ion secondary battery, a
method of forming a protecting layer on the surface of an electrode
(negative electrode) has been proposed (see, for example, Japanese
Patent Laid-Open No. 7-220759, International Publication No. WO
97/01870 and Japanese Patent Laid-Open No. 11-54147).
[0005] The protecting layer is basically formed by depositing a
particle. The particle size and particle size distribution, in this
case, have an effect upon the characteristics of the battery such
as rate characteristics and charge-discharge cycle characteristics,
and safety such as internal short circuit caused by dendrite
formation. In a conventional protecting layer, an inorganic
particle is basically used. When an inorganic particle is used, it
is difficult to control the particle size and particle size
distribution. Therefore, a small-size inorganic particle is
selected to form a protecting layer having a uniform thickness.
[0006] The conventional protecting layer is basically formed by
depositing an inorganic particle.
[0007] The conventional protecting layer is basically formed by
depositing an inorganic particle. However, an inorganic particle
has a wide particle size distribution. To form a protecting layer
having uniform thickness, a small-size inorganic particle is
selected.
[0008] As a lithium ion secondary battery, one having a pair of
electrodes facing each other and a separator interposed between the
electrodes is known. In a process for manufacturing such a lithium
ion secondary battery, when the separator is broken by e.g.,
convexoconcave of an electrode surface, short circuit occurs
between the electrodes facing each other, generating heat. To
prevent occurrence of such a problem, a technique of forming a
protecting layer using an inorganic particle and an organic
particle on a surface of an active-material containing layer has
been proposed (see, for example, Japanese Patent Laid-Open No.
7-220759, International Publication No. WO97/01870 and Japanese
Patent Laid-Open No. 11-54147).
SUMMARY OF THE INVENTION
First Object of the Invention
[0009] When the protecting layer is formed of a particle having a
small particle size, dendrite tends to grow. Because of this,
satisfactory rate characteristics, charge-discharge cycle
characteristics and safety are rarely obtained.
[0010] A first invention was made in view of the problems of the
conventional technique mentioned above and is directed to providing
an electrode for a lithium ion secondary battery capable of forming
a lithium ion secondary battery sufficiently suppressed in
occurrence of internal short circuit due to dendrite growth and
excellent in rate characteristics and charge-discharge cycle
characteristics, and providing a lithium ion secondary battery
using the same.
Second Object of the Invention
[0011] Since a protecting layer using an inorganic particle does
not have a shutdown function, the safety during a heat-up time is
not sufficient. Even if the separator has a shutdown function, a
possibility of internal short circuit due to shrinkage
(contraction) of the separator still remains. Although safety can
be improved by increasing the thickness of the protecting layer,
characteristics of a battery such as impedance and rate
characteristics tend to deteriorate. On the other hand, even with a
protecting layer using an organic particle, it is difficult to
obtain a shutdown function and suppress occurrence of internal
short circuit due to shrinkage of the protecting layer during a
heat-up time, at the same time.
[0012] A second invention was made in view of the problems of the
conventional technique mentioned above and is directed to providing
an electrode for a lithium ion secondary battery capable of forming
a lithium ion secondary battery excellent in safety during a
heat-up time and in rate characteristics, and sufficiently reduced
in impedance, and providing a lithium ion secondary battery using
the same.
Third Object of the Invention
[0013] When the protecting layer is formed of a particle having
small particle size, an electrolytic solution rarely penetrates
into the protecting layer, causing a problem of deterioration in
characteristics such as rate characteristics of a battery. A
protecting layer formed of an inorganic particle does not have a
shutdown function. The safety of such a protecting layer during a
heat-up time is not sufficient.
[0014] On the other hand, even with a protecting layer using an
organic particle, it is difficult to provide a shutdown function
and suppress occurrence of internal short circuit due to shrinkage
(contraction) of the protecting layer during a heat-up time, at the
same time. Furthermore, when a shutdown function is provided to the
protecting layer by using an organic particle, if a lithium ion
secondary battery is used repeatedly, impedance may probably
increase.
[0015] The invention was made in view of the problems of the
conventional technique mentioned above and is directed to providing
an electrode for a lithium ion secondary battery capable of forming
a lithium ion secondary battery excellent in safety during a
heat-up time and in rate characteristics and sufficiently
suppressed in increase of impedance at the time of repeated use,
and providing a lithium ion secondary battery using the same.
Fourth Object of the Invention
[0016] When a protecting layer is formed of an inorganic particle,
in order to obtain the protecting layer uniform in thickness, the
size of the inorganic particle must be set to be sufficiently
smaller than the thickness of the layer. In this case, the
interspace between particles becomes narrow, preventing ion
migration in an electrolytic solution. As a result, rate
characteristics tend to deteriorate. Note that, when the thickness
of the protecting layer is nonuniform, impedance disadvantageously
increases.
[0017] On the other hand, when a protecting layer is formed of an
organic particle, if the inner temperature of a battery increases
due to overcharge, etc., the separator shrinks (contracts). In
addition, since the protecting layer formed of an organic particle
does not have sufficient heat resistance, the protecting layer may
not sufficiently work. In the circumstances, it has been desired to
develop a lithium ion secondary battery capable of sufficiently
suppressing deterioration of rate characteristics and short circuit
even at high temperatures.
[0018] The invention was made in view of the problems of the
conventional technique mentioned above and is directed to a lithium
ion secondary battery capable of sufficiently suppressing
deterioration of rate characteristics and short circuit even at
high temperatures.
First Invention
[0019] To attain the first object, the first invention provides an
electrode for a lithium ion secondary battery having a collector,
an active-material layer formed on the collector and a protecting
layer formed on the active-material layer, in which the protecting
layer contains an organic particle formed of poly(methyl
methacrylate) having a crosslinked structure, and the organic
particle has an average particle size (D50) of 0.5 to 4.0
.mu.m.
[0020] According to the electrode for a lithium ion secondary
battery, since a protecting layer is formed of an organic particle,
which is formed of poly(methyl methacrylate) having a crosslinked
structure and has an average particle size (D50) of 0.5 to 4.0
.mu.m, on the surface of the electrode, a lithium ion secondary
battery sufficiently suppressed in occurrence of internal short
circuit due to dendrite growth and excellent in rate
characteristics and charge-discharge cycle characteristics can be
formed, compared to an electrode having a protecting layer formed
of a conventional inorganic particle or an organic particle other
than the organic particle specified by the present application.
Furthermore, use of organic particle specified above provides
excellent shutdown function of the protecting layer at high
temperatures and can improve safety at high temperatures, compared
to an electrode formed of an inorganic particle.
[0021] In the electrode for a lithium ion secondary battery
according to the first invention, it is preferred that the organic
particle has a shape satisfying the conditions expressed by the
following expression (1):
1.00.ltoreq.(major-axis length/minor-axis length).ltoreq.1.30
(1).
[0022] The organic particles satisfying the conditions of the
expression (1) are likely to uniformly align in the protecting
layer with no space between particles. As a result, growth of
dendrite in the thickness direction of the protecting layer is
inhibited; occurrence of internal short circuit can be more
sufficiently suppressed; and the rate characteristics and
charge-discharge cycle characteristics of a lithium ion secondary
battery can be further improved.
[0023] The first invention also provides a lithium ion secondary
battery having a positive electrode and a negative electrode, in
which at least one of the positive electrode and the negative
electrode is an electrode having a collector, an active-material
layer formed on the collector and a protecting layer formed on the
active-material layer; the protecting layer contains an organic
particle formed of a poly(methyl methacrylate) having a crosslinked
structure; and the organic particle has an average particle size
(D50) of 0.5 to 4.0 .mu.m.
[0024] According to the lithium ion secondary battery, since it has
a negative electrode and/or positive electrode having a protecting
layer formed of an organic particle, which is formed of poly(methyl
methacrylate) having a crosslinked structure and has an average
particle size (D50) of 0.5 to 4.0 .mu.m, on the surface, occurrence
of internal short circuit due to dendrite growth is sufficiently
suppressed and excellent rate characteristics and excellent
charge-discharge cycle characteristics can be obtained, compared to
an electrode having a protecting layer formed of a conventional
inorganic particle or an organic particle except the organic
particle specified by the present application. Furthermore, use of
organic particle specified above provides excellent shutdown
function of the protecting layer at high temperatures and can
improve safety at high temperatures, compared to an electrode
formed of an inorganic particle.
[0025] In the lithium ion secondary battery of the first invention,
it is preferred that the organic particle has a shape satisfying
the conditions expressed by the following expression (1):
1.00.ltoreq.(major-axis length/minor-axis length).ltoreq.1.30
(1).
[0026] The organic particles satisfying the conditions of the
expression (1) are likely to uniformly align in the protecting
layer with no space between particles. As a result, growth of
dendrite in the thickness direction of the protecting layer is
inhibited; occurrence of internal short circuit can be more
sufficiently suppressed; and the rate characteristics and
charge-discharge cycle characteristics of a lithium ion secondary
battery can be further improved.
[0027] Furthermore, in the lithium ion secondary battery of the
first invention, it is preferred that at least the negative
electrode is an electrode having the collector, the active-material
layer and the protecting layer.
[0028] Occurrence of internal short circuit due to dendrite growth
can be more sufficiently suppressed by providing a protecting layer
to a negative electrode rather than a positive electrode. This is
because dendrite is likely to grow particularly when a material,
such as graphite, having a low potential is used as an active
material of a negative electrode.
Second Invention
[0029] To attain the second object, the second invention provides
an electrode for a lithium ion secondary battery having a
collector, an active-material layer formed on the collector and a
protecting layer formed on the active-material layer, in which the
protecting layer contains an organic particle and an inorganic
particle; the organic particle has a melting temperature of 100 to
200.degree. C.; the organic particle and the inorganic particle
each have an average particle size (D50) of 0.10 to 4.0 .mu.m; and
a ratio of a content of the organic particle relative to a content
of the inorganic particle in the protecting layer is 1:1 to 1:4, in
terms of mass.
[0030] Since the protecting layer contains the organic particle and
inorganic particle specified above in the aforementioned ratio, a
sufficient shutdown function can be provided without degrading
characteristics of a battery such as impedance and rate
characteristics; at the same time, shrinkage during a heat-up time
can be sufficiently suppressed. More specifically, the function the
protecting layer provides shutdown since the organic particle melts
during a heat-up time; however the inorganic particle does not melt
and remains as it is. Since the shape of the protecting layer can
be maintained in this manner, internal short circuit can be
prevented. Therefore, according to the electrode for a lithium ion
secondary battery of the second invention, which has a protecting
layer on the surface, a lithium ion secondary battery excellent in
safety during a heat-up time and in rate characteristics, and
sufficiently reduced in impedance can be formed.
[0031] In the electrode for a lithium ion secondary battery of the
second invention, it is preferred that the organic particle is a
particle formed of polyethylene. By virtue of this, more
satisfactory shutdown function of the protecting layer can be
obtained, and a lithium ion secondary battery further improved in
safety during a heat-up time can be formed.
[0032] The second invention also provides a lithium ion secondary
battery having a positive electrode and a negative electrode, in
which at least one of the positive electrode and the negative
electrode is an electrode having a collector, an active-material
layer formed on the collector and a protecting layer formed on the
active-material layer; the protecting layer contains an organic
particle and an inorganic particle; the organic particle has a
melting temperature of 100 to 200.degree. C.; the organic particle
and the inorganic particle each have an average particle size (D50)
of 0.10 to 4.0 .mu.m; and a ratio of a content of the organic
particle relative to a content of the inorganic particle in the
protecting layer is 1:1 to 1:4, in terms of mass.
[0033] Since the protecting layer contains the organic particle and
inorganic particle specified above in the aforementioned ratio, a
sufficient shutdown function can be obtained without degrading
characteristics of a battery such as impedance and rate
characteristics; at the same time, shrinkage during a heat-up time
can be sufficiently suppressed. More specifically, the protecting
layer provides shutdown since the organic particle melts during a
heat-up time; however the inorganic particle does not melt and
remains as it is. Since the shape of the protecting layer can be
maintained in this manner, internal short circuit can be prevented.
Therefore, according to the lithium ion secondary battery of the
second invention, which has the negative electrode and/or positive
electrode having the protecting layer on the surface, excellent
safety during a heat-up time and excellent rate characteristics can
be obtained, and further impedance can be sufficiently reduced.
[0034] In the lithium ion secondary battery of the second
invention, it is preferred that the organic particle is a particle
formed of polyethylene. By virtue of this, more satisfactory
shutdown function of the protecting layer can be obtained and the
safety of lithium ion secondary battery during a heat-up time can
be further improved.
[0035] Furthermore, in the lithium ion secondary battery of the
second invention, it is preferred that at least the negative
electrode is an electrode having the collector, the active-material
layer and the protecting layer.
[0036] The safety during a heat-up time can be further improved by
providing the protecting layer to a negative electrode rather than
a positive electrode. This is because growth of lithium dendrite
particularly on the surface of the negative electrode
active-material can be likely to be suppressed.
Third Invention
[0037] To attain the third object, the third invention provides an
electrode for a lithium ion secondary battery having a collector,
an active-material layer formed on the collector and a protecting
layer formed on the active-material layer, in which the protecting
layer contains a low-melting point organic particle having a
melting temperature of 100 to 200.degree. C., and a high-melting
point organic particle having a melting temperature of 300.degree.
C. or more.
[0038] Since the protecting layer mentioned above contains two
types of organic particles different in melting temperature, a
sufficient shutdown function can be obtained without degrading
characteristics of a battery such as impedance and rate
characteristics; at the same time, shrinkage during a heat-up time
can be sufficiently suppressed. More specifically, the protecting
layer provides shutdown since the low-melting point organic
particle first melts during a heat-up time; however the
high-melting point organic particle does not melt and remains as it
is. Since the shape of the protecting layer can be maintained in
this manner, internal short circuit can be prevented. Therefore,
according to the electrode for a lithium ion secondary battery of
the third invention, which has a protecting layer on the surface, a
lithium ion secondary battery excellent in safety during a heat-up
time and in rate characteristics and sufficiently suppressed in
increase of impedance at the time of repeated use can be
formed.
[0039] In the electrode for a lithium ion secondary battery of the
third invention, it is preferred that the low-melting point organic
particle is a particle formed of at least one material selected
from the group consisting of polyethylene, polypropylene and
poly(methyl methacrylate). By virtue of this, more satisfactory
shutdown function of the protecting layer can be obtained, and a
lithium ion secondary battery more excellent in safety during a
heat-up time and more sufficiently suppressed in increase of
impedance at the time of repeated use can be formed.
[0040] In the electrode for a lithium ion secondary battery of the
third invention, it is preferred that the high-melting point
organic particle is a particle formed of at least one material
selected from the group consisting of polyimide and
polytetrafluoroethylene. By virtue of this, the protecting layer is
more sufficiently suppressed in shrinkage during a heat-up time and
a lithium ion secondary battery further improved in safety during a
heat-up time can be formed.
[0041] In the electrode for a lithium ion secondary battery of the
third invention, it is preferred that the ratio of a content of the
low-melting point organic particle relative to a content of the
high-melting point organic particle in the protecting layer is 1:1
to 1:4, in terms of mass. Since the protecting layer contains the
low-melting point organic particle and the high-melting point
organic particle in the aforementioned ratio, providing a shutdown
function to the protecting layer, and suppressing occurrence of
internal short circuit due to shrinkage of the protecting layer
during a heat-up time can be both attained at a high level.
[0042] The third invention also provides a lithium ion secondary
battery having a positive electrode and a negative electrode, in
which at least one of the positive electrode and the negative
electrode is an electrode having a collector, an active-material
layer formed on the collector and a protecting layer formed on the
active-material layer; and the protecting layer contains a
low-melting point organic particle having a melting temperature of
100 to 200.degree. C. and a high-melting point organic particle
having a melting temperature of 300.degree. C. or more.
[0043] Since the protecting layer contains two types of organic
particles different in melting temperature, a sufficient shutdown
function can be obtained without degrading characteristics of a
battery such as impedance and rate characteristics; at the same
time, shrinkage during a heat-up time can be sufficiently
suppressed. More specifically, the protecting layer provides
shutdown since the low-melting point organic particle first melts
during a heat-up time; however the high-melting point organic
particle does not melt and remains as it is. Since the shape of the
protecting layer can be maintained in this manner, internal short
circuit can be prevented. Therefore, according to the lithium ion
secondary battery of the third invention, which has a negative
electrode and/or positive electrode having the protecting layer on
the surface, excellent safety during a heat-up time, excellent rate
characteristics can be obtained and an increase of impedance at the
time of repeated use can be sufficiently suppressed.
[0044] In the lithium ion secondary battery of the third invention,
it is preferred that the low-melting point organic particle is a
particle formed of at least one material selected from the group
consisting of polyethylene, polypropylene and poly(methyl
methacrylate). By virtue of this, the protecting layer can be
provided with more satisfactory shutdown function, and the lithium
ion secondary battery can be further improved in safety during a
heat-up time and more sufficiently suppressed in an increase of
impedance at the time of repeated use.
[0045] In the lithium ion secondary battery of the third invention,
it is preferred that the high-melting point organic particle is a
particle formed of at least one material selected from the group
consisting of polyimide and polytetrafluoroethylene. By virtue of
this, the protecting layer is more sufficiently suppressed in
shrinkage during a heat-up time and the lithium ion secondary
battery can be further improved in safety during a heat-up
time.
[0046] In the lithium ion secondary battery of the third invention,
it is preferred that the ratio of a content of the low-melting
point organic particle relative to a content of the high-melting
point organic particle in the protecting layer is 1:1 to 1:4, in
terms of mass. Since the protecting layer contains a low-melting
point organic particle and a high-melting point organic particle in
the aforementioned ratio, providing a shutdown function to the
protecting layer, and suppressing occurrence of internal short
circuit due to shrinkage of the protecting layer during a heat-up
time can be both attained at a high level. Therefore, safety of the
lithium ion secondary battery during a heat-up time can be further
improved.
[0047] Furthermore, in the lithium ion secondary battery of the
third invention, it is preferred that at least the negative
electrode is an electrode having the collector, the active-material
layer and the protecting layer.
[0048] The safety during a heat-up time can be further improved by
providing a protecting layer to a negative electrode rather than a
positive electrode. This is because growth of dendrite particularly
on the surface of the negative electrode active-material can be
suppressed.
Fourth Invention
[0049] To attain the fourth object, the fourth invention provides a
lithium ion secondary battery having a pair of electrodes facing
each other and a separator interposed between the electrodes, in
which at least one of the electrodes has a protecting layer, an
active-material containing layer and a collector sequentially from
the separator; the protecting layer contains a silicone resin
particle having at least one of structural units represented by
RSiO.sub.1.5 and R.sub.2SiO (in the formula, R represents an alkyl
group having 1 to 6 carbon atoms or a phenyl group).
[0050] In the fourth invention, it is possible to suppress
deterioration of rate characteristics, at the same time,
sufficiently suppress short circuit even at high temperatures. The
reasons for this are unknown; however, the present inventors
consider as follows. However, the reasons are not limited to the
followings. Since the protecting layer contains the aforementioned
silicone resin particle, even if the thickness of the protecting
layer is reduced to 1 to 6 fold as small as the particle size of
the silicone resin particle, the protecting layer having a
relatively uniform thickness can be formed. Therefore, it is not
necessary to reduce the particle size of the particle constituting
the protecting layer to be sufficiently small compared to the
thickness of the protecting layer in order to form a protecting
layer having a uniform thickness. Therefore, as the particle
constituting the protecting layer, a particle having a relatively
large particle size close to the thickness of the protecting layer
can be used. By virtue of this, the interspace between particles is
widened, and thus, ions of an electrolytic solution can easily
migrate between particles (resistance value of ion migration is
reduced). Therefore, the deterioration of rate characteristics is
conceivably suppressed. Furthermore, since the protecting layer
contains the silicone resin particle, the heat resistance of the
protecting layer improves. Consequently, the function of the
protecting layer can be maintained even at high temperatures (for
example, 400.degree. C.). Therefore, short circuit is conceivably
sufficiently suppressed even at high temperatures.
[0051] It is preferred that the silicone resin particle has an
average particle size of 0.3 to 6.0 .mu.m. In this case, uniformity
in thickness of the protecting layer can be further improved.
Therefore, an ion of an electrolytic solution can more easily
migrate between particles and the deterioration of rate
characteristics can be further suppressed.
[0052] It is preferred that the protecting layer has a thickness of
0.3 to 6.0 .mu.m. In this case, the deterioration of rate
characteristics can be further suppressed.
[0053] It is preferred that an aspect ratio, which is a ratio of a
major-axis length relative to a minor-axis length of the silicone
resin particle, is 1.00 to 1.50. In this case, sizes of particles
tend to be equal and thus the uniformity of thickness of the
protecting layer can be easily improved. Therefore the
deterioration of rate characteristics can be easily suppressed.
[0054] It is preferred that the silicone resin particle is a
polymethylsilsesquioxane particle. In this case, the deterioration
of rate characteristics can be further suppressed; at the same
time, short circuit can be further suppressed even at high
temperatures.
Advantages of the First Invention
[0055] As described above, according to the first invention, it is
possible to provide an electrode for a lithium ion secondary
battery capable of forming a lithium ion secondary battery
sufficiently suppressed in occurrence of internal short circuit due
to dendrite growth and excellent in rate characteristics and
charge-discharge cycle characteristics, and provide a lithium ion
secondary battery using the same.
Advantages of the Second Invention
[0056] According to the second invention, it is possible to provide
an electrode for a lithium ion secondary battery capable of forming
a lithium ion secondary battery excellent in safety during a
heat-up time and in rate characteristics, and sufficiently reduced
in impedance, and provide a lithium ion secondary battery using the
same.
Advantages of the Third Invention
[0057] According to the third invention, it is possible to provide
an electrode for a lithium ion secondary battery capable of forming
a lithium ion secondary battery excellent in safety during a
heat-up time and in rate characteristics, and sufficiently
suppressed in increase of impedance at the time of repeated use,
and provide a lithium ion secondary battery using the same.
Advantages of the Fourth Invention
[0058] According to the fourth invention, it is possible to provide
a lithium ion secondary battery capable of sufficiently suppressing
the deterioration of rate characteristics and short circuit even at
high temperatures.
BRIEF DESCRIPTION OF THE DRAWINGS
[0059] FIG. 1 is a front view of a preferred embodiment of a
lithium ion secondary battery according to the present
invention;
[0060] FIG. 2 is a schematic sectional view of the lithium ion
secondary battery shown in FIG. 1, taken along the X-X line in FIG.
1;
[0061] FIG. 3 is a schematic sectional view of a preferred
embodiment of a basic structure of a negative electrode of a
lithium ion secondary battery;
[0062] FIG. 4 is a schematic sectional view of a preferred
embodiment of a basic structure of a positive electrode of a
lithium ion secondary battery;
[0063] FIG. 5 is a partially cutaway perspective view of another
preferred embodiment of a lithium ion secondary battery of the
present invention;
[0064] FIG. 6 is a schematic sectional view of the lithium ion
secondary battery shown in FIG. 5, taken along the YZ plane;
and
[0065] FIG. 7 is a schematic sectional view of the lithium ion
secondary battery according to an embodiment of the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0066] Preferred embodiments of the present invention will be more
specifically described below referring to the accompanying
drawings. Note that, in the drawings, the same symbols are used to
designate the same or corresponding elements and any further
explanation is omitted for brevity's sake. Furthermore, positional
relationship like up and down and right and left is the same of
that shown in the drawings, if not otherwise specified.
Furthermore, the dimensional ratios are not limited to those shown
in the drawings.
[0067] FIG. 1 is a front view of a preferred embodiment of a
lithium ion secondary battery according to the first to third
inventions.
[0068] FIG. 2 is a schematic sectional view of a lithium ion
secondary battery 1 shown in FIG. 1, taken along the X-X line.
[0069] As shown in FIG. 1 and FIG. 2, the lithium ion secondary
battery 1 is constituted essentially of a power generation element
60, which consists of a plate-form negative electrode 10 and a
plate-form positive electrode 20 facing each other, and a
plate-form separator 40 arranged closely between the negative
electrode 10 and the positive electrode 20, an electrolytic
solution (non-aqueous electrolytic solution in the embodiment)
containing a lithium ion, a case 50 housing these airtight, a lead
12 for the negative electrode, one of the ends of which is
electrically connected to the negative electrode 10 and the other
end of which protrudes out of the case 50, and a lead 22 for the
positive electrode, one of the ends of which is electrically
connected to the positive electrode 20 and the other end of which
protrudes out of the case 50.
[0070] Note that, the "negative electrode" used herein is an
electrode defined based on the polarity at the time the battery is
electrically discharged, more specifically referred to an electrode
releasing electrons by an oxidation reaction during electric
discharge. Furthermore, the "positive electrode" is an electrode
defined based on the polarity at the time the battery is
electrically discharged, more specifically, referred to an
electrode receiving electrons by a reductive reaction during
electric discharge.
[0071] Furthermore, FIG. 3 and FIG. 4 each are a schematic
sectional view of a preferred embodiment of an electrode for a
lithium ion secondary battery according to the first to third
inventions. More specifically, FIG. 3 is a schematic sectional view
of an embodiment of a basic structure of the negative electrode 10
of the lithium ion secondary battery 1. FIG. 4 is a schematic
sectional view of an embodiment of a basic structure of the
positive electrode 20 of the lithium ion secondary battery 1.
[0072] As shown in FIG. 3, the negative electrode 10 is constituted
of a collector 16, a negative electrode active-material layer 18
formed on the collector 16 and a protecting layer 30 formed on the
negative electrode active-material layer 18. Furthermore, as shown
in FIG. 4, the positive electrode 20 is constituted of a collector
26, a positive electrode active-material layer 28 formed on the
collector 26 and a protecting layer 30 formed on the positive
electrode active-material layer 28.
[0073] The protecting layer 30 according to the first invention is
a layer formed of an organic particle formed of poly(methyl
methacrylate) (PMMA) having a crosslinked structure and having an
average particle size (D50) of 0.5 to 4.0 .mu.m containing.
[0074] Furthermore, the protecting layer 30 according to the second
invention is a layer containing an organic particle and an
inorganic particle. In the protecting layer 30, the organic
particle has a melting temperature of 100 to 200.degree. C., the
organic particle and the inorganic particle each have an average
particle size (D50) of 0.10 to 4.0 .mu.m. Furthermore, in the
protecting layer 30, the ratio of a content of the organic particle
relative to a content of the inorganic particle is 1:1 to 1:4, in
terms of mass.
[0075] Furthermore, the protecting layer 30 according to the third
invention is a layer containing a low-melting point organic
particle having melting temperature of 100 to 200.degree. C. and a
high-melting point organic particle having a melting temperature of
300.degree. C. or more.
[0076] The collector 16 and the collector 26 are not particularly
limited as long as they are good conductive materials sufficiently
mediating migration of a charge to the negative electrode
active-material layer 18 and the positive electrode active-material
layer 28, and a collector used in a known lithium ion secondary
battery can be used. For example, foil of a metal such as copper
and aluminum is mentioned as the collector 16 and the collector 26,
respectively.
[0077] The negative electrode active-material layer 18 of the
negative electrode 10 is essentially formed of a negative electrode
active-material and a binder. Note that, it is preferred that the
negative electrode active-material layer 18 further contains a
conductive auxiliary.
[0078] The negative electrode active-material is not particularly
limited as long as occlusion and release of a lithium ion,
desorption and insertion (intercalation) of a lithium ion, or
doping and de-doping of a lithium ion can be reversibly carried
out, and a known negative electrode active-material can be used.
Examples of such a negative electrode active-material include a
carbon material such as natural graphite, synthetic graphite,
hardly-graphitized carbon, easily-graphitized carbon and
low-temperature baked carbon, a metal such as Al, Si and Sn,
capable of chemically reacting with lithium, an amorphous compound
principally formed of an oxide such as SiO, SiO.sub.2, SiO.sub.x,
and SnO.sub.2, lithium titanate (Li.sub.4Ti.sub.5O.sub.12) and
TiO.sub.2.
[0079] As a binder to be used in the negative electrode 10, a known
binder can be used without any particular limitation. Examples
thereof include fluorine resins such as polyvinylidene fluoride
(PVDF), polytetrafluoroethylene (PTFE), a
tetrafluoroethylene-hexafluoropropylene copolymer (FEP), a
tetrafluoroethylene-perfluoroalkylvinylether copolymer (PFA), an
ethylene-tetrafluoroethylene copolymer (ETFE), polychlorotrifluoro
ethylene (PCTFE), an ethylene-chlorotrifluoroethylene copolymer
(ECTFE) and polyvinyl fluoride (PVF). To the binder, a functional
group such as carboxylic acid may be added in order to mutually
bind components such as an active material particle and a
conductive auxiliary added if necessary, more sufficiently, and
bind these components to a collector, more sufficiently.
[0080] Furthermore, other than the aforementioned binders, for
example, vinylidene fluoride based fluorine rubber such as
vinylidene fluoride-hexafluoropropylene based fluorine rubber
(VDF-HFP based fluorine rubber) may be used as a binder.
[0081] Furthermore, examples of the binder that may be used other
than the aforementioned binders include polyethylene,
polypropylene, polyethylene terephthalate, aromatic polyamide,
cellulose and a derivative thereof, styrene-butadiene rubber,
isoprene rubber, butadiene rubber and ethylene-propylene rubber.
Examples of the cellulose derivative include sodium
carboxymethylcellulose, hydroxyethylcellulose, cellulose acetate,
cellulose nitrate and cellulose sulfate. Furthermore, a
thermoplastic elastomer polymer may be used such as a
styrene-butadiene-styrene block copolymer and a hydrogenated
material thereof, a styrene-ethylene-butadiene-styrene copolymer,
and a styrene-isoprene-styrene block copolymer and a hydrogenated
material thereof. Furthermore, use may be made of syndiotactic
1,2-polybutadiene, an ethylene-vinyl acetate copolymer and a
propylene-.alpha.-olefin (carbon atoms: 2 to 12) copolymer, etc.
Alternatively, a conductive polymer may be used.
[0082] The conductive auxiliary to be used, if necessary, is not
particularly limited and a known conductive auxiliary can be used.
Examples of the conductive auxiliary include carbon black, a carbon
material, powder of a metal such as copper, nickel, stainless steel
and iron, a mixture of a carbon material and a metal powder, and a
conductive oxide such as ITO.
[0083] The content of the negative electrode active-material in the
negative electrode active-material layer 18 is preferably 80 to 98%
by mass based on the total amount of negative electrode
active-material layer 18, and more preferably 85 to 97% by mass.
When the content of the active material is less than 80% by mass,
energy density tends to decrease, compared to the case where the
content falls within the aforementioned range. When the content
exceeds 98% by mass, adhesive force is insufficient and cycle
characteristics tend to deteriorate, compared to the case where the
content falls within the aforementioned range.
[0084] The positive electrode active-material layer 28 of the
positive electrode 20 is constituted essentially of a positive
electrode active-material and a binder. Note that, it is preferred
that the positive electrode active-material layer 28 further
contains a conductive auxiliary.
[0085] The positive electrode active-material is not particularly
limited as long as occlusion and release of a lithium ion,
desorption and insertion (intercalation) of a lithium ion or doping
and de-doping of a lithium ion can be reversibly carried out and a
known positive electrode active-material can be used. Examples of
such a positive electrode active-material include lithium cobaltate
(LiCoO.sub.2), lithium nickelate (LiNiO.sub.2), lithium-manganese
spinel (LiMn.sub.2O.sub.4), a compound metal oxide represented by
the general formula:
LiNi.sub.xCo.sub.yMn.sub.zM.sub.aO.sub.2
where x+y+z+a=1 (0.ltoreq.x.ltoreq.1, 0.ltoreq.y.ltoreq.1,
0.ltoreq.z.ltoreq.1, 0.ltoreq.a.ltoreq.1), M is at least one
element selected from Al, Mg, Nb, Ti, Cu, Zn and Cr, and compound
metal oxides such as a lithium vanadium compound
(LiV.sub.2O.sub.5), an olivine-type LiMPO.sub.4 (where M is at
least one element selected from Co, Ni, Mn, Fe, Mg, Nb, Ti, Al and
Zr or VO) and lithium titanate (Li.sub.4Ti.sub.5O.sub.12).
[0086] As the binder to be used in the positive electrode 20, the
same ones to be used in the negative electrode 10 can be used.
Furthermore, as the conductive auxiliary to be used, if necessary,
in the positive electrode 20, the same conductive auxiliaries to be
used in the negative electrode 10 can be used.
[0087] The content of the positive electrode active-material in the
positive electrode active-material layer 28 is preferably 80 to 98%
by mass based on the total amount of positive electrode
active-material layer 28, and more preferably 85 to 97% by mass.
When the content of the active material is less than 80% by mass,
energy density tends to decrease, compared to the case where the
content falls within the aforementioned range. When the content
exceeds 98% by mass, adhesive force is insufficient and cycle
characteristics tend to deteriorate, compared to the case where the
content falls within the aforementioned range.
[0088] (Protecting Layer 30 According to First Invention)
[0089] Each of the protecting layers 30 according to the first
invention of the negative electrode 10 and the positive electrode
20 is a layer containing an organic particle formed of poly(methyl
methacrylate) having a crosslinked structure and having an average
particle size (D50) of 0.5 to 4.0 .mu.m. The protecting layer 30
may be a layer consisting only of the organic particle or may be a
layer containing an organic particle and other materials such as a
binder.
[0090] The average particle size (D50) of the organic particle
formed of a poly(methyl methacrylate) having a crosslinked
structure is 0.5 to 4.0 .mu.m, preferably 0.8 to 3.5 .mu.m, and
more preferably 1.0 to 3.0 .mu.m. When the average particle size
(D50) is less than 0.5 .mu.m, it becomes difficult to sufficiently
suppress dendrite growth, with the result that sufficient
charge-discharge cycle characteristics and safety cannot be
obtained. On the other hand, when the average particle size (D50)
exceeds 4.0 .mu.m, sufficient rate characteristics cannot be
obtained. The average particle size (D50) of the organic particle
is calculated based on the measurement data by an apparatus (name
of apparatus: HRA manufactured by Micro Track) for measuring
particle-size distribution based on a laser diffraction-dispersion
method.
[0091] Furthermore, the organic particle preferably has a shape
satisfying the conditions represented by the following equation
(1):
1.00.ltoreq.(major-axis length/minor-axis length).ltoreq.1.30
(1).
[0092] A value of major-axis length/minor-axis length of the
organic particle is preferably 1.00 to 1.30, more preferably 1.00
to 1.20, and particularly preferably, 1.00 to 1.10. The organic
particle is preferably close to a true sphere. When the value of
major-axis length/minor-axis length exceeds 1.30, rate
characteristics and charge-discharge cycle characteristics tend to
deteriorate.
[0093] The value of major-axis length/minor-axis length can be
measured by use of an electron microscope. More specifically, in
the present invention, the major-axis length/minor-axis length
value is obtained by calculation as an average of, major-axis
length/minor-axis length values of arbitrarily selected 10 organic
particles under observation by an electron microscope.
[0094] Characteristics of an organic particle formed of a
poly(methyl methacrylate) having a crosslinked structure, such as
weight average molecular weight, hardness and degree of
crosslinking, have an effect upon the characteristics of the
protecting layer 30. The weight average molecular weight of the
organic particle preferably falls within the range of 100,000 to
1,000,000. When the weight average molecular weight is less than
100,000, an electrolytic solution does not sufficiently permeate
into the protecting layer 30. As a result, the amounts of
electrolytic solution held by the negative electrode
active-material layer 18 and the positive electrode active-material
layer 28 decrease. Because of this, impedance tends to increase.
When the weight average molecular weight is larger than 1,000,000,
the organic particle swells and the size of the organic particle
increases excessively. As a result, the distance between the
positive electrode 20 and the negative electrode 10 increases. In
this case, impedance also tends to increase.
[0095] Hardness of the organic particle can be evaluated based on
the hardness measured by a durometer in accordance with JIS K7215
test method. Hardness measured by a durometer preferably falls
within the range of 70 to 130, and more preferably within the range
of 90 to 130. When hardness by a durometer is less than 70,
mechanical strength of the protecting layer 30 decreases and the
protecting layer 30 tends to be destroyed by dendrite grown in an
electrode surface. When hardness by a durometer is larger than 130,
the surface of an electrode is scratched during the
expansion/contraction of the electrode in the surface
charge-discharge cycle. The charge-discharge characteristics tend
to deteriorate.
[0096] The degree of crosslinking of poly(methyl methacrylate)
constituting an organic particle can be estimated by determining a
gel fraction. The gel fraction is obtained by soaking an organic
particle in a solvent of 25.degree. C. for 24 hours and measuring a
reduction of mass. The gel fraction preferably falls within the
range of 98.0 to 100%. When the gel fraction is less than 98.0%,
the mechanical strength of the protecting layer 30 decreases and
the protecting layer 30 tends to be destroyed by dendrite grown in
an electrode surface.
[0097] As the binder to be used, if necessary, in the protecting
layer 30, for example, styrene-butadiene rubber, sodium
carboxymethylcellulose, polyvinyl alcohol, PVDF and PTFE are
mentioned. Of these, styrene-butadiene rubber and sodium
carboxymethylcellulose are preferred in view of adhesiveness to an
electrode and controlling of viscosity of a coating solution.
[0098] As the material that can be used in the protecting layer 30
other than an organic particle and a binder, for example, an
inorganic material such as ceramic is mentioned. Any material may
be used, if necessary, for suppressing dendrite, as long as it has
high resistance and is not involved in desorption/insertion of a
lithium ion during a charge-discharge time.
[0099] When the protecting layer 30 contains a binder other than
the organic particle, the content of the binder is preferably not
less than 1% by mass based on the total amount of protecting layer
30, and more preferably 1.5 to 30% by mass, in view of removal of
particles from the protecting layer 30 and permeability of an
electrolytic solution,
[0100] The thickness of the protecting layer 30 is preferably 0.5
to 4.0 .mu.m, and more preferably 1.0 to 4.0 .mu.m. When the
thickness of the protecting layer 30 is less than 0.5 .mu.m, the
effect of suppressing dendrite growth tends to reduce. When the
thickness exceeds 4.0 .mu.m, rate characteristics tend to
deteriorate.
[0101] Note that, the protecting layer 30 of the negative electrode
10 and the protecting layer 30 of the positive electrode 20 may
have the same structure or different structures.
[0102] (Protecting Layer 30 According to Second Invention)
[0103] Each of the protecting layers 30 of the negative electrode
10 and the positive electrode 20, according to the second invention
is a layer containing an organic particle and an inorganic
particle. Note that the protecting layer 30 may be a layer
consisting only of an organic particle and an inorganic particle or
a layer containing an organic particle, an inorganic particle and
other materials such as a binder.
[0104] The organic particle is not particularly limited as long as
it has a melting temperature of 100 to 200.degree. C. and an
average particle size (D50) of 0.10 to 4.0 As a material for the
organic particle, for example, polyethylene (PE), polypropylene
(PP), poly(methyl methacrylate) (PMMA), an ethylene-acrylic acid
copolymer (EA), polyvinyl chloride (PVC), polyester and
polyurethane are mentioned. Of these, the organic particle is
preferably a particle formed of polyethylene, polypropylene or
poly(methyl methacrylate), for more sufficiently obtaining the
effect of the invention.
[0105] The melting temperature of the organic particle is 100 to
200.degree. C., preferably 102 to 190.degree. C., and more
preferably 105 to 180.degree. C.
[0106] When the melting temperature of the organic particle falls
within the above range, it is possible to obtain a satisfactory
shutdown function of the protecting layer 30 during a heat-up
time.
[0107] Furthermore, the average particle size (D50) of the organic
particle is 0.10 to 4.0 .mu.m, preferably 0.20 to 3.3 .mu.m, and
more preferably 0.30 to 3.5 .mu.m. When the average particle size
(D50) is less than 0.10 .mu.m, permeability of an electrolytic
solution into the protecting layer 30 and the active-material layer
is inhibited and characteristics of a battery such as rate
characteristics deteriorate. On the other hand, when the average
particle size (D50) exceeds 4.0 .mu.m, impedance increases.
[0108] The inorganic particle is not particularly limited as long
as it has an average particle size (D50) of 0.10 to 4.0 .mu.m. As
the material for the inorganic particle, for example, alumina,
silica and titania are mentioned. Of these inorganic particles, a
particle formed of alumina or silica is preferred for sufficiently
obtaining the effect of the invention.
[0109] Furthermore, the average particle size (D50) of the
inorganic particle is 0.10 to 4.0 .mu.m, preferably 0.13 to 3.0 and
more preferably 0.15 to 2.0 .mu.m. When the average particle size
(D50) is less than 0.10 .mu.m, permeation of an electrolytic
solution into the protecting layer 30 and the active-material layer
is inhibited and characteristics of a battery such as rate
characteristics deteriorate. On the other hand, when the average
particle size (D50) exceeds 4.0 .mu.m, impedance increases.
[0110] The average particle size (D50) of the organic particle and
the inorganic particle is calculated based on the measurement data
by an apparatus (type: HRA manufactured by Micro Track) for
measuring particle-size distribution based on a laser
diffraction-dispersion method.
[0111] Furthermore, the ratio of the average particle size (D50) of
the organic particle relative to the average particle size (D50) of
the inorganic particle (organic-particle average particle size
(D50): inorganic particle average particle size (D50)) is
preferably 1:2 to 4:1, and more preferably 1:1 to 3:1. When the
inorganic particle average particle size (D50) relative to the
organic particle average particle size (D50) is larger than the
above range, which means that the size of the inorganic particle is
large, permeability of an electrolytic solution decreases and
impedance tends to increase. On the other hand, when the inorganic
particle average particle size (D50) relative to the organic
particle average particle size (D50) is smaller than the above
range, which means that the size of the inorganic particle is
small, internal short circuit is likely to occur during a
temperature raising test. In addition, since the size of the
organic particle increases, the surface area decreases, decreasing
the rate of a shutdown reaction. Consequently, good results may not
be likely to be obtained in an overcharge test.
[0112] In the protecting layer 30, the ratio of the organic
particle content and the inorganic particle content (organic
particle content: inorganic particle content) is 1:1 to 1:4 (mass
ratio), preferably 1:1.3 to 1:3.2, and more preferably 1:1.5 to
1:3.0. The organic particle content relative to the inorganic
particle content is smaller than the above range, sufficient
shutdown function of the protecting layer 30 cannot be obtained,
with the result that safety during a heat-up time decreases. On the
other hand, when the organic particle content relative to the
inorganic particle content is larger than the above range, the
protecting layer 30 tends to shrink during a heat-up time, with the
result that it becomes difficult to sufficiently suppress
occurrence of internal short circuit.
[0113] As the binder to be used, if necessary, in the protecting
layer 30, for example, styrene-butadiene rubber, sodium
carboxymethylcellulose, polyvinyl alcohol, PVDF and PTFE are
mentioned. Of these, styrene-butadiene rubber and sodium
carboxymethylcellulose are preferred in view of adhesiveness to an
active-material layer and adjustment of viscosity of a coating
solution.
[0114] As the other materials that can be used in the protecting
layer 30 other than an organic particle, an inorganic particle and
a binder, for example, an inorganic material such as ceramic is
mentioned. Any material may be used if necessary for suppressing
dendrite, as long as it has high resistance and is not involved in
desorption/insertion of a lithium ion during a charge-discharge
time.
[0115] When the protecting layer 30 contains a binder other than an
organic particle and an inorganic particle, the content of the
binder, preferably not more than 10% by mass based on the total
amount of protecting layer 30, and more preferably not more than 8%
by mass, for maintaining a shut down effect in an overcharge test
and an effect of suppressing occurrence of short circuit due to
shrinkage of a separator during a temperature raising test by the
protecting layer 30.
[0116] The thickness of the protecting layer 30 is preferably 0.5
to 4.0 .mu.m, and more preferably 1.0 to 3.5 .mu.m. When the
thickness of the protecting layer 30 is less than 0.5 .mu.m, the
effect of suppressing occurrence of internal short circuit by the
protecting layer 30 tends to be insufficient. When the thickness
exceeds 4.0 .mu.m, rate characteristics deteriorate and impedance
tends to increase.
[0117] Note that, the protecting layer 30 of the negative electrode
10 and the protecting layer 30 of the positive electrode 20 may
have the same structure or different structures.
[0118] (Protecting Layer 30 According to Third Invention)
[0119] Each of the protecting layers 30 according to the third
invention in the negative electrode 10 and the positive electrode
20 is a layer containing a low-melting point organic particle
having a melting temperature of 100 to 200.degree. C. and a
high-melting point organic particle having a melting temperature of
300.degree. C. or more. Note that, the protecting layer 30 may be a
layer consisting only of a low-melting point organic particle and a
high-melting point organic particle, or formed of a low-melting
point organic particle, a high-melting point organic particle and
other materials such as a binder.
[0120] The low-melting point organic particle is not particularly
limited as long as it has a melting temperature of 100 to
200.degree. C. As a material for the low-melting point organic
particle, for example, polyethylene (PE), polypropylene (PP),
poly(methyl methacrylate) (PMMA), an ethylene-acrylic acid
copolymer (EA) and polyvinyl chloride (PVC) are mentioned. Of
these, the low-melting point organic particle, for sufficiently
obtaining the effect of the invention, is preferably a particle
formed of at least one material selected from the group consisting
of polyethylene, polypropylene and poly(methyl methacrylate).
[0121] The melting temperature of the low-melting point organic
particle is 100 to 200.degree. C., preferably 102 to 190.degree.
C., and more preferably 105 to 180.degree. C. When the melting
temperature of the low-melting point organic particle is less than
100.degree. C., the temperature at which the protecting layer 30
provides shutdown is excessively low, unwanted shut down takes
place when a lithium ion secondary battery is repeatedly used, with
the result that impedance disadvantageously increases. On the other
hand, when the melting temperature of the low-melting point organic
particle exceeds 200.degree. C., sufficient shutdown function of
the protecting layer 30 cannot be obtained and thermal runaway
disadvantageously proceeds during a heat-up time.
[0122] Furthermore, the average particle size (D50) of the
low-melting point organic particle is preferably 0.10 to 6.0 .mu.m,
more preferably 0.30 to 5.0 .mu.m, and particularly preferably 0.50
to 4.0 .mu.m. When the average particle size (D50) is less than
0.10 .mu.m, permeation of an electrolytic solution into the
protecting layer 30 and the active-material layer is inhibited,
characteristics of a battery such as rate characteristics tend to
deteriorate. On the other hand, when the average particle size
(D50) exceeds 6.0 .mu.m, impedance tends to increase.
[0123] The high-melting point organic particle is not particularly
limited as long as it has a melting temperature of 300.degree. C.
or more. As a material for the high-melting point organic particle,
for example, poly benzimidazole (PBI), polyimide (PT),
polytetrafluoroethylene (PTFE), a
polytetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA),
polyetherketone (PEK), polyetheretherketone (PEEK),
polyethersulfone (PES), polyamideimide (PAT) and polyetherimide
(PET) are mentioned. Of these, the high-melting point organic
particle is preferably a particle formed of at least one material
selected from the group consisting of polyamideimide, polyimide and
polytetrafluoroethylene, for sufficiently obtaining the effect of
the invention.
[0124] Note that, a polyimide (PT) particle used in Example 3-5
(described later) can be prepared by an isocyanate method known as
a manufacturing method in, for example, Japanese Patent Laid-Open
No. 59-108030 and Japanese Patent Laid-Open No. 60-221425.
[0125] The high-melting point organic particle has a melting
temperature of 300.degree. C. or more, preferably 300 to
350.degree. C., and more preferably 350 to 400.degree. C. When the
high-melting point organic particle has a melting temperature of
less than 300.degree. C., the protecting layer 30 shrinks during a
heat-up time and a problem of internal short circuit tends to
occur.
[0126] Furthermore, the average particle size (D50) of the
high-melting point organic particle is preferably 0.30 to 6.0
.mu.m, more preferably 0.50 to 5.0 .mu.m, and particularly
preferably 1.0 to 4.0 .mu.m. When the average particle size (D50)
is less than 0.30 .mu.m, permeation of an electrolytic solution
into the protecting layer 30 and the active-material layer is
inhibited and characteristics of a battery such as rate
characteristics tends to deteriorate. On the other hand, when an
average particle size (D50) exceeds 6.0 .mu.m, impedance tends to
increase.
[0127] The average particle size (D50) of the low-melting point
organic particle and the high melting point organic particle is
calculated based on the measurement data by an apparatus (name of
apparatus: BRA manufactured by Micro Track) for measuring
particle-size distribution based on a laser diffraction-dispersion
method.
[0128] Furthermore, the ratio of the average particle size (D50) of
the low-melting point organic particle and the average particle
size (D50) of the high-melting point organic particle (the average
particle size (D50) of the low-melting point organic particle: the
average particle size (D50) of the high-melting point organic
particle) is preferably 1:1.5 to 1:8, and more preferably 1:2 to
1:6. When the average particle size (D50) of the low-melting point
organic particle relative to the average particle size (D50) of the
high-melting point organic particle is smaller than the above
range, a low-melting point organic particle tends to melt early
during a heat-up time. Thus, internal impedance tends to increase.
On the other hand, the average particle size (D50) of the
low-melting point organic particle relative to the average particle
size (D50) of the high-melting point organic particle is larger
than the above range, the specific surface area of a particle
decreases and thereby a shut down reaction is delayed.
Consequently, it seems very possible that good results are not
obtained in an overcharge test.
[0129] In the protecting layer 30, the ratio of the content of the
low-melting point organic particle and the content of the
high-melting point organic particle (a low-melting point organic
particle content: high-melting point organic particle content) is
preferably 1:1 to 1:4 (mass ratio), more preferably 1:1.3 to 1:3.2,
and particularly preferably, 1:1.5 to 1:3.0. The low-melting point
organic particle content relative to the high-melting point organic
particle content is less than the above range, the shutdown
function of the protecting layer 30 is lowered and safety during a
heat-up time tend to decrease. On the other hand, when the
low-melting point organic particle content relative to the
high-melting point organic particle content is larger than the
above range, the protecting layer 30 is likely to shrink during a
heat-up time, and an effect of suppressing occurrence of internal
short circuit tends to reduce.
[0130] As the binder to be used, if necessary, in the protecting
layer 30, for example, styrene-butadiene rubber, sodium
carboxymethylcellulose, polyvinyl alcohol, PVDF and PTFE are
mentioned. Of these, styrene-butadiene rubber and sodium
carboxymethylcellulose are preferred in view of adhesiveness to
active-material layer and controlling of viscosity of a coating
solution.
[0131] As the material that can be used in the protecting layer 30
other than the low-melting point organic particle, the high-melting
point organic particle and the binder, for example, an inorganic
material such as ceramic is mentioned. Any material may be used if
necessary for suppressing dendrite, as long as it has high
resistance and is not involved in desorption/insertion of a lithium
ion during a charge-discharge time.
[0132] When the protecting layer 30 contains a binder other than
the low-melting point organic particle and the high-melting point
organic particle, the content of the binder is preferably not more
than 10% by mass based on the total amount of protecting layer 30,
and more preferably not more than 8% by mass, for maintaining a
shut down effect during an overcharge test.
[0133] The thickness of the protecting layer 30 is preferably 0.3
to 6.0 .mu.m, and more preferably 0.5 to 5.0 .mu.m. When the
thickness of the protecting layer 30 is less than 0.3 .mu.m, the
effect of suppressing occurrence of internal short circuit by the
protecting layer 30 tends to be insufficient. When the thickness
exceeds 6.0 .mu.m, rate characteristics deteriorate and impedance
tends to increase.
[0134] Note that, the protecting layer 30 of the negative electrode
10 and the protecting layer 30 of the positive electrode 20 may
have the same structure or different structures.
[0135] The collector 26 of the positive electrode 20 is
electrically connected to an end of the positive-electrode lead 22,
for example, formed of aluminum. The other end of the
positive-electrode lead 22 extends out of the case 50. On the other
hand, the collector 16 of the negative electrode 10 is also
electrically connected to an end of the negative-electrode lead 12,
for example, made of cupper or nickel. The other end of the
negative-electrode lead 12 extends out of the case 50.
[0136] The separator 40 arranged between the negative electrode 10
and the positive electrode 20 is not particularly limited as long
as it is formed of a porous material having ion transmissivity and
electronic insulating property and a separator used in a known
lithium ion secondary battery can be used. Example thereof include
a laminate of films formed of polyethylene, polypropylene or
polyolefin, a stretched film of a mixture of the polymers or
nonwoven cloth of a fiber made of at least one selected from the
group consisting of cellulose, polyester and polypropylene.
[0137] The inner space of the case 50 is filled with an
electrolytic solution (not shown) and a part of the electrolytic
solution is contained in the negative electrode 10, the positive
electrode 20 and the separator 40. As the electrolytic solution, a
non-aqueous electrolytic solution dissolving a lithium salt in an
organic solvent is used. Examples of the lithium salt include
LiPF.sub.6, LiClO.sub.4, LiBF.sub.4, LiAsF.sub.6,
LiCF.sub.3SO.sub.3, LiCF.sub.3CF.sub.2SO.sub.3,
LiC(CF.sub.3SO.sub.2).sub.3, LiN(CF.sub.3SO.sub.2).sub.2,
LiN(CF.sub.3CF.sub.2SO.sub.2).sub.2,
LiN(CF.sub.3SO.sub.2)(C.sub.4F.sub.9SO.sub.2) and
LiN(CF.sub.3CF.sub.2CO).sub.2. Note that, these salts may be used
singly or in combination with two types or more. Furthermore, the
electrolytic solution may be in a gel state by adding a polymer or
the like.
[0138] Furthermore, as the organic solvent, a solvent used in a
known lithium ion secondary battery can be used. Preferable
examples thereof include propylene carbonate, ethylene carbonate
and diethyl carbonate. These may be used singly or in a mixture of
two types or more in an appropriate ratio.
[0139] The case 50 is formed, as shown in FIG. 2, of a pair of
films (a first film 51 and a second film 52) facing each other. The
overlapped edges of the films facing each other are sealed with an
adhesive agent or by heat seal to form a seal portion 50A.
[0140] The films constituting the first film 51 and the second film
52 are films having flexibility. These films are not particularly
limited as long as they have flexibility. For ensuring sufficient
mechanical strength and lightness of the case and effectively
preventing invasion of water and air into the case 50 from the
outside of the case 50 and dissipation of electrolyte components
from the interior of the case 50 to the exterior of the case 50, it
is preferred to have at least an innermost layer formed of a
polymer in contact with power generation element 60 and a metal
layer, which is arranged in the opposite surface of the innermost
layer in contact with the power generation element.
[0141] A portion of the negative-electrode lead 12 in contact with
the sealing portion 50A is coated with an insulating material 14 in
order to prevent contact between the negative-electrode lead 12 and
the metal layer of the case 50. A portion of the positive-electrode
lead 22 in contact with the sealing portion 50A is coated with an
insulating material 24 in order to prevent contact between the
positive-electrode lead 22 and the metal layer of the ease 50.
Furthermore, the insulating materials 14, 24 also play a role in
improving tightness between the innermost layer of the case 50 and
the leads 12, 22.
[0142] Next, a method for manufacturing the aforementioned lithium
ion secondary battery 1 will be described.
[0143] First, the negative electrode 10 and the positive electrode
20 are prepared. In preparing the negative electrode 10, a method
for forming the negative electrode active-material layer 18 is not
particularly limited. For example, components of the negative
electrode 10 as described above are mixed and dispersed in a
solvent capable of dissolving a binder to prepare a coating
solution (e.g., slurry or paste) for forming a negative electrode
active-material layer. The solvent is not particularly limited as
long as it can dissolve a binder. For example,
N-methyl-2-pyrrolidone, N,N-dimethylformamide and water can be used
depending upon the type of binder to be used.
[0144] Next, the coating solution for forming the negative
electrode active-material layer is applied onto the surface of the
collector 16, dried, if necessary, rolled or the like, to form the
negative electrode active-material layer 18 on the collector 16. A
method for applying the coating solution for forming the negative
electrode active-material layer to the surface of the collector 16
is not particularly limited and appropriately determined depending
upon the material and shape of the collector 16. Examples of the
coating method include a metal mask printing method, an
electrostatic coating method, dip coating method, a spray coating
method, a roll coating method, a doctor blade method, a gravure
coating method and a screen printing method.
[0145] Next, the protecting layer 30 is formed on the negative
electrode active-material layer 18.
[0146] (Method for Forming the Protecting Layer 30 According to the
First Invention)
[0147] When the protecting layer 30 according to the first
invention is a layer containing an organic particle, a binder and
other materials if needed, first, the aforementioned components of
the protecting layer 30 are mixed and dispersed in a solvent
capable of dissolving the binder to prepare a coating solution
(e.g., slurry or paste) for forming a protecting layer. The solvent
is not particularly limited as long as it dissolves the binder and
does not dissolve the organic particle. Examples thereof include
water, a compound having a hydroxyl group such as methanol,
ethanol, isopropyl alcohol, amyl alcohol, ethylene glycol, glycerin
and cyclohexanol; a ketone such as acetone, methyl ethyl ketone,
methyl isobutyl ketone and cyclohexanone; an ester such as ethyl
acetate, propyl acetate, butyl propionate, butyl butyrate and ethyl
lactate, a hydrocarbon such as toluene, xylene, n-butane,
cyclohexane and cyclopentane, and an ether such as ethyl ether,
butyl ether, ethylpropyl ether, allyl ether, tetrahydrofuran and
phenyl ether. These solvents can be used depending upon the type of
binder to be used.
[0148] Next, the coating solution for forming the protecting layer
is applied onto the surface of the negative electrode
active-material layer 18 and dried to form the protecting layer 30
on the negative electrode active-material layer 18. At this time,
if necessary, the protecting layer 30 may be subjected to a
treatment such as a press treatment.
[0149] Examples of the press treatment include roll press by means
of e.g., a calender roll or flat-plate press. In the present
invention, roll press is desirably employed since it is
advantageous in forming a highly dense negative electrode
active-material layer 18. If there is a risk of e.g., deformation
of the negative electrode 10 by performing processing at a high
pressure, the processing may be performed by low-pressure thermal
press. When thermal press is employed, it is desirable that thermal
press is appropriately applied in consideration of heat resistance.
Note that, at this time, a preferable temperature is generally 80
to 180.degree. C. The pressure during the pressurization process is
preferably controlled such that the porosity of the protecting
layer 30 becomes preferably 20 to 40% and more preferably 25 to
35%. The porosity is expressed by:
[1-(density of the protecting layer 30/true density of the
protecting layer 30)].times.100.
A method for applying a coating solution for forming a protecting
layer onto the surface of the negative electrode active-material
layer 18 is not particularly limited and may be appropriately
determined depending upon the material and shape of the negative
electrode active-material layer 18. As the coating method, the same
method as employed in applying the coating solution for forming the
negative electrode active-material layer may be mentioned.
[0150] Furthermore, when the protecting layer 30 consists only of
an organic particle, the protecting layer 30 can be formed by
dispersing the organic particle in a solvent to prepare a
dispersion solution (coating solution), applying the solution,
drying and, if necessary, pressing or the like, in the same manner
as mentioned above.
[0151] (Method for Forming the Protecting Layer 30 According to the
Second Invention)
[0152] When the protecting layer 30 according to the second
invention is a layer containing an organic particle, an inorganic
particle, a binder and, if necessary, other materials, first,
components constituting the protecting layer 30 as mentioned above
are mixed and dispersed in a solvent capable of dissolving a binder
to prepare a coating solution (e.g., slurry or paste) for forming a
protecting layer. The solvent is not particularly limited as long
as it can dissolve the binder and does not dissolve the organic
particle. Examples thereof include water, a compound having a
hydroxyl group such as methanol, ethanol, isopropyl alcohol, amyl
alcohol, ethylene glycol, glycerin and cyclohexanol; a ketone such
as acetone, methyl ethyl ketone, methyl isobutyl ketone and
cyclohexanone; an ester such as ethyl acetate, propyl acetate,
butyl propionate, butyl butyrate and ethyl lactate, a hydrocarbon
such as toluene, xylene, n-butane, cyclohexane and cyclopentane,
and an ether such as ethyl ether, butyl ether, ethyipropyl ether,
ally! ether, tetrahydrofuran and phenyl ether. These solvents can
be used depending upon the type of binder to be used.
[0153] Next, a coating solution for forming the protecting layer is
applied onto the surface of the negative electrode active-material
layer 18 and dried to form the protecting layer 30 on the negative
electrode active-material layer 18. At this time, it is preferred
that a treatment such as rolling may not be applied to the
protecting layer 30 for maintaining the shapes of the organic
particle and the inorganic particle; however, if the shape of the
organic particle is not affected, a rolling treatment may be
applied. A method for applying the coating solution for forming a
protecting layer onto the surface of the negative electrode
active-material layer 18 is not particularly limited and may be
appropriately determined depending upon the material and shape of
the negative electrode active-material layer 18. As the coating
method, the same method as employed in applying the coating
solution for forming the negative electrode active-material layer
may be mentioned.
[0154] Furthermore, when the protecting layer 30 consists only of
the organic particle and the inorganic particle, the protecting
layer 30 can be formed by dispersing the organic particle and the
inorganic particle in a solvent to prepare a dispersion solution
(coating solution), applying the solution, drying and, if
necessary, pressing or the like, in the same manner as mentioned
above.
[0155] (Method for Forming Protecting Layer 30 According to the
Third Invention)
[0156] When the protecting layer 30 according to the third
invention is a layer containing a low-melting point organic
particle, a high-melting point organic particle, a binder and, if
necessary, other materials, first, components constituting the
protecting layer 30 as mentioned above are mixed and dispersed in a
solvent capable of dissolving a binder to prepare a coating
solution (e.g., slurry or paste) for forming a protecting layer.
The solvent is not particularly limited as long as it can dissolve
a binder and does not dissolve the low-melting point organic
particle and the high-melting point organic particle. Examples
thereof include water, a compound having a hydroxyl group such as
methanol, ethanol, isopropyl alcohol, amyl alcohol, ethylene
glycol, glycerin and cyclohexanol; a ketone such as acetone,
methylethyl ketone, methylisobutyl ketone and cyclohexanone; an
ester such as ethyl acetate, propyl acetate, butyl propionate,
butyl butyrate and ethyl lactate, a hydrocarbon such as toluene,
xylene, n-butane, cyclohexane and cyclopentane and an ether such as
ethyl ether, butyl ether, ethylpropyl ether, allyl ether,
tetrahydrofuran and phenyl ether. These solvents can be used
depending upon the type of binder to be used.
[0157] Next, a coating solution for forming the protecting layer is
applied onto the surface of the negative electrode active-material
layer 18 and dried to form the protecting layer 30 on the negative
electrode active-material layer 18. At this time, it is preferred
that a treatment such as a rolling may not be applied to the
protecting layer 30 for maintaining the shapes of the low-melting
point organic particle and the high-melting point organic particle;
however, if the shape of the organic particle is not affected, a
rolling treatment may be applied. A method for applying the coating
solution for forming a protecting layer onto the surface of the
negative electrode active-material layer 18 is not particularly
limited and may be appropriately determined depending upon the
material and shape of the negative electrode active-material layer
18. As the coating method, the same method as employed in applying
the coating solution for forming the negative electrode
active-material layer may be mentioned.
[0158] Furthermore, when the protecting layer 30 consists only of
the low-melting point organic particle and the high-melting point
organic particle, the protecting layer 30 can be formed by
dispersing the organic particles in a solvent to prepare a
dispersion solution (coating solution), applying the solution,
drying and, if necessary, pressing or the like, in the same manner
as mentioned above.
[0159] Furthermore, the positive electrode 20 can be prepared in
the same manner as in the negative electrode 10.
[0160] After the negative electrode 10 and the positive electrode
20 are prepared as mentioned above, the negative-electrode lead 12
and positive-electrode lead 22 are electrically connected to the
negative electrode 10 and the positive electrode 20,
respectively.
[0161] Next, the separator 40 is arranged between the negative
electrode 10 and the positive electrode 20 in contact with them
(preferably in an unbonded state) to complete formation of the
power generation element 60 (a laminate formed of the negative
electrode 10, the separator 40 and the positive electrode 20
stacked sequentially in this order). At this time, the separator 40
is arranged such that it comes into contact with the surface F2 of
the negative electrode 10 on the side of the protecting layer 30
and the surface F2 of the positive electrode 20 on the side of the
protecting layer 30.
[0162] Next, the edge portions of the first film 51 and the second
film 52 overlapped are sealed by an adhesive agent or by heat seal
to prepare the case 50. At this time, a part is allowed to remain
unsealed in order to ensure an opening portion for introducing the
power generation element 60 into the case 50 (performed in the
later step). In this way, the case 50 having an opening portion is
obtained.
[0163] Into the case 50 having an opening portion, the power
generation element 60 to which the negative-electrode lead 12 and
the positive-electrode lead 22 are electrically connected is
inserted and further an electrolytic solution is injected.
Subsequently, the opening portion of the case 50 is sealed in the
state where a part of the negative-electrode lead 12 and a part of
the positive-electrode lead 22 are inserted in the case 50 to
complete formation of the lithium ion secondary battery 1.
[0164] In the foregoing, preferred embodiments of the present
invention have been described in detail; however, the present
invention is not limited to the above embodiments.
[0165] For example, in the embodiments above, the case where the
negative electrode 10 and the positive electrode 20 both have the
protecting layer 30 is described; however, only one of the negative
electrode 10 and the positive electrode 20 may have the protecting
layer 30. Note that, it is preferred that at least the negative
electrode 10 has the protecting layer 30 in order to produce the
effect of the invention more sufficiently.
[0166] Furthermore, in the embodiments above, a lithium ion
secondary battery 1 having a single negative electrode 10 and a
single positive electrode 20 is described; however, a lithium ion
secondary battery may have at least two negative electrodes 10 and
at least two positive electrodes 20 with a single separator 40
always arranged between them. Furthermore, the shape of the lithium
ion secondary battery 1 is not limited to the one shown in FIG. 1.
For example, a cylindrical shape may be employed.
[0167] Furthermore, the lithium ion secondary battery of the
present invention can be used as a power source of automatic
micromachines and IC cards, etc. and also used as a dispersed power
source arranged on or within a printed substrate.
[0168] Other preferred embodiments of a lithium ion secondary
battery according to the first to the third inventions will be
described.
[0169] FIG. 5 shows a partially cutaway perspective view of a
lithium ion secondary battery 100 according to another preferred
embodiment according to the first to the third inventions.
Furthermore, FIG. 6 is a sectional view of the lithium ion
secondary battery shown in FIG. 5 taken along the YZ plane. The
lithium ion secondary battery 100 according to this embodiment, as
shown in FIG. 5 and FIG. 6, is constituted essentially of a
laminate structure 85, a case (outer package) 50 housing the
laminate structure 85 airtight, a negative-electrode lead 12 and a
positive-electrode lead 22 for connecting between the laminate
structure 85 and the outer portion of the case 50.
[0170] The laminate structure 85, as shown in FIG. 6, is formed of
a two-surface coated negative electrode 130, a separator 40, a two
surface-coated positive electrode 140, a separator 40, a
two-surface coated negative electrode 130, a separator 40, a two
surface-coated positive electrode 140, a separator 40 and a
two-surface coated negative electrode 130 stacked in this order
from the top.
[0171] The two-surface coated negative electrode 130 has a
collector (negative electrode collector) 16, two negative electrode
active-material layers 18 formed on both surfaces of the collector
16 and two protecting layers 30 formed on each of the negative
electrode active-material layers 18. The two-surface coated
negative electrode 130 is laminated such that the protecting layer
30 is allowed to be in contact with the separator 40.
[0172] Furthermore, the two surface-coated positive electrode 140
has a collector (positive electrode collector) 26 and two positive
electrode active-material layers 28 formed on both surface of the
collector 26. The two surface-coated positive electrode 140 is
laminated such that the positive electrode active-material layer 28
is allowed to be in contact with the separator 40.
[0173] The inner space of the case 50 is filled with an
electrolytic solution (not shown) and partly contained in the
negative electrode active-material layer 18, the positive electrode
active-material layer 28, the protecting layer 30 and the separator
40.
[0174] The edges of the collectors 16, 26 are formed into
tongue-shaped portions 16a, 26a each extending outward, as shown in
FIG. 5. Furthermore, the negative-electrode lead 12 and the
positive-electrode lead 22, as shown in FIG. 5, protrude from the
case 50 by way of the sealing portion 50b. The end of the lead 12
within the case 50 is welded to each of the tongue-shaped portions
16a of three collectors 16. The lead 12 is electrically connected
to each of the negative electrode active-material layers 18 via the
corresponding collector 16. On the other hand, the end of the lead
22 within the case 50 is welded to each of the tongue-shaped
portions 26a of two collectors 16. The lead 22 is electrically
connected to each of the positive electrode active-material layer
28 via the corresponding collector 26.
[0175] Furthermore, the portions of the leads 12, 22 sandwiched by
the sealing portions 50b of the case 50 are coated with an
insulating material 14, 24 such as a resin, as shown in FIG. 5, to
increase sealing performance. Furthermore, the lead 12 and the lead
22 are arranged at a distance in the direction orthogonal with the
lamination direction of a laminate structure 85.
[0176] The case 50 is, as shown in FIG. 5, formed by folding a
rectangular flexible sheet 51C virtually in half lengthwise so as
to sandwich the laminate structure 85 vertically at the top and the
bottom. Of the edge portions of the sheet 51 C folded, three edges
excluding a folded portion 50a are sealing portions 50b, which are
adhered by heat seal or with an adhesive agent to enclose the
laminate structure 85 airtight within the case. Furthermore, the
case 50 seals the leads 12, 22 by adhering to insulating materials
14, 24 at the sealing portion 50b.
[0177] In the lithium ion secondary battery 100 as shown in FIG. 5
and FIG. 6, the collectors 16, 26, active-material layers 18, 28,
protecting layer 30, separator 40, electrolytic solution, leads 12,
22, insulating materials 14, 24 and the case 50 are formed of the
same materials as used in the lithium ion secondary battery 1 shown
in FIG. 1 to FIG. 4.
[0178] Note that, in the lithium ion secondary battery 100 as shown
in FIG. 5 and FIG. 6, the laminate structure 85 has 4 secondary
battery elements each serving as a single cell, in other words, 4
combinations of negative electrode/separator/positive electrode;
however, the number of combination may have more than 4 or 3 or
less.
[0179] Furthermore, in the embodiment above, it is preferred that
the two outermost layers are each formed of a two-surface coated
negative electrode 130; however, even if either one or both of the
two outermost layers may be formed of a two-layered (one-surface
coated) negative electrode(s), the present invention can be carried
out.
[0180] Furthermore, in the embodiment above, it is preferred that
two outermost layers are each formed of a negative electrode;
however, even if the two outermost layers are formed of a positive
electrode and a negative electrode or a positive electrode and a
positive electrode, the present invention can be carried out.
[0181] Furthermore, in the embodiment above, a structure where the
protecting layer 30 is provided only to a negative electrode is
shown as an example; however, the protecting layer 30 may be
provided also to a positive electrode. Furthermore, the protecting
layer 30 is not provided to the negative electrode and may be
provided only to a positive electrode. Moreover, in the embodiment
above, a structure where the protecting layer 30 is provided to
both surfaces of a two-surface coated negative electrode is shown
as an example; however, the protecting layer 30 may be provided one
of the negative electrode active-material layers.
[0182] (Lithium Ion Secondary Battery According to the Fourth
Invention)
[0183] First, the lithium ion secondary battery 200 according to
the embodiment will be described referring to FIG. 7.
[0184] The lithium ion secondary battery 200 essentially has a
laminate 230, a case 250 housing the laminate 230 airtight and a
pair of leads 260, 262 connected to the laminate 230.
[0185] The laminate 230 has a pair of positive electrode 210 and
negative electrode 220 facing each other and separator 218 arranged
between the positive electrode 210 and the negative electrode 220.
The positive electrode 210 has, sequentially from the side of the
separator 218, a positive electrode protecting layer 216, a
positive electrode active-material containing layer 214 and a
positive electrode collector 212. The negative electrode 220 has,
sequentially from the side of the separator 218, a negative
electrode protecting layer 226, a negative electrode
active-material containing layer 224 and a negative electrode
collector 222. The positive electrode protecting layer 216 and a
negative electrode protecting layer 226 (hereinafter, described
sometimes as protecting layers 216, 226) are in contact with both
sides of the separator 218, respectively.
[0186] The positive electrode collector 212 may employ, for
example, an aluminum foil or a nickel foil. The negative electrode
collector 222 may employ, for example, a copper foil or a nickel
foil.
[0187] The positive electrode active-material containing layer 214
and the negative electrode active-material containing layer 224
contain an active-material particle, a binder and, if necessary, a
conductive auxiliary. The positive electrode active-material
containing layer 214 has a thickness of, for example, 50 to 140
.mu.m. The negative electrode active-material containing layer 224
has a thickness of, for example, 40 to 130 .mu.m.
[0188] As the positive electrode active-material particle, for
example, mention may be made of a lithium oxide containing at least
one metal selected from the group consisting of Co, Ni and Mn, such
as LiMO.sub.2 (M represents Co, Ni or Mn),
LiCo.sub.xNi.sub.1-xO.sub.2, LiMn.sub.2O.sub.4,
LiCo.sub.xNi.sub.yMn.sub.1-x-yO.sub.2 (where, x and y each exceed 0
and less than 1). LiCo.sub.xNi.sub.yMn.sub.1-x-yO.sub.2 is
particularly preferable.
[0189] As the negative electrode active-material particle, for
example, mention is made of a carbon particle capable of absorbing
or desorbing (releasing) a lithium ion (intercalate/deintercalate,
or doping/dedoping) such as graphite, hardily-graphitized carbon,
easily-graphitized carbon and low-temperature baked carbon, a
particle of a composite material of carbon and a metal, a metal
particle capable of reacting with lithium, such as Al, Si and Sn,
and a particle containing lithium titanate
(Li.sub.4Ti.sub.5O.sub.12) or the like.
[0190] The binder is not particularly limited as long as it can
bind the aforementioned active-material particle and a conductive
auxiliary to a collector. A known binder can be used. Examples of
the binder include a fluorine resin such as polyvinylidene fluoride
(PVDF), polytetrafluoroethylene (PTFE) and a mixture of
styrene-butadiene rubber (SBR) and a water soluble polymer
(carboxymethylcellulose (CMC), polyvinyl alcohol, sodium
polyacrylate, dextrin, gluten, etc.).
[0191] Examples of the conductive auxiliary include carbon black, a
carbon material, a micropowder of a metal such as copper, nickel,
stainless steel and iron, a mixture of a carbon material and a
metal micropowder and a conductive oxide such as ITO.
[0192] The positive electrode protecting layer 216 and the negative
electrode protecting layer 226 are each a porous insulating layer.
The positive electrode protecting layer 216 and the negative
electrode protecting layer 226 each contains a silicone resin
particle as a Si-containing organic particle, preferably contains a
binder as mentioned above. The silicone resin particle is easily
permeable with an electrolytic solution. The silicone resin
particle has at least one of structural units represented by
RSiO.sub.1.5 and R.sub.2SiO (in the formula, R represents an alkyl
group having 1 to 6 carbon atoms or a phenyl group) and a siloxane
bond (Si--O--Si). Preferable examples of the alkyl group include a
methyl group, an ethyl group, a propyl group, a butyl group, a
pentyl group and a hexyl group. The silicone resin particle
particularly preferably contains polymethylsilsesquioxane particle
represented by the above formula, RSiO.sub.1.5, where R is a methyl
group. Note that, R in the structural unit may mutually differ
between the structural units or two Rs of the above formula
R.sub.2SiO may have mutually different functional groups.
[0193] The silicone resin particle has a melting point of
preferably 150.degree. C. or more, more preferably 200.degree. C.
or more and further preferably, 300.degree. C. or more, for
preventing short circuit during an overcharge test and suppressing
generation of gas due to thermal decomposition.
[0194] The aspect ratio of the silicone resin particle, which is a
ratio of the major-axis diameter to the minor-axis diameter, is
preferably 1.00 to 1.50, more preferably 1.00 to 1.40 and further
preferably 1.00 to 1.30. When the aspect ratio exceeds 1.50,
permeability of an electrolytic solution decreases. As a result,
rate characteristics tend to deteriorate. The aspect ratio is
defined by a value obtained by dividing the major-axis diameter b
of a silicone resin particle by the minor-axis diameter a, (b/a),
and can be obtained by calculation as an average (b/a) value of
arbitrarily chosen 10 silicone resin particles under an electron
microscope.
[0195] The average particle size of the silicone resin particle is
preferably 0.3 to 6.0 .mu.m, more preferably 0.5 to 5.0 .mu.m and
further preferably, 1.0 to 4.0 .mu.m. When the average particle
size exceeds 6.0 .mu.m, the thicknesses of the protecting layers
216, 226 exceed 6.0 .mu.m. As a result, the ion migration distance
within each of the protecting layers 216, 226 increases. Therefore,
the resistance value of ion migration increases and rate
characteristics tends to deteriorate. When the average particle
size is less than 0.3 .mu.m, to ensure a short circuit prevention
function while maintaining the thicknesses of the protecting layers
216, 226 at a certain level, it is necessary to increase the number
of layers of a particle to some extent. As a result, the interspace
between particles tends to be narrow. The average particle size of
the silicone resin particle can be defined by, for example, D50,
which is 50%-diameter in a volume-based particle size distribution.
The volume-based particle size distribution of the silicone resin
particle can be easily measured by an apparatus (for example: Micro
Track HRA (trade name) manufactured by Micro Track) for measuring
particle-size distribution based on a laser diffraction-dispersion
method.
[0196] The thicknesses of the positive electrode protecting layer
216 and the negative electrode protecting layer 226 are each
preferably 0.3 to 6.0 .mu.m, more preferably 0.5 to 5.0 .mu.m, and
further preferably, 1.0 to 4.0 .mu.m. When the thickness exceeds
6.0 .mu.m, an ion migration distance within the protecting layers
216, 226 increases. As a result, a resistance value of ion
migration within the protecting layers 216, 226 increases and rate
characteristics tend to deteriorate. When the thickness is less
than 0.3 .mu.m, the short circuit prevention function of the
protecting layers 216, 226 tends to be rarely fulfilled. The
positive electrode protecting layer 216 and the negative electrode
protecting layer 226 are preferably arranged so as not to be
overlapped with each other. This is because ions can easily migrate
between the particles.
[0197] The separator 218 is sufficient if it is made of a porous
material having electrical insulation properties. Example thereof
include a single layer or a laminate of films formed of
polyethylene, polypropylene or polyolefin and a stretched film of a
mixture of these resins or nonwoven cloth of a fiber made of at
least one component selected from the group consisting of
cellulose, polyester and polypropylene.
[0198] The case 250 houses the laminate 230 and an electrolytic
solution airtight. The case 250 is not particularly limited as long
as it can suppress, e.g., leakage of an electrolytic solution
outside and invasion of water or the like from the outside into the
lithium ion secondary battery 200. For example, as the case 250, a
metal laminate film formed by a metal foil 252 having coating of a
polymer film 254 on both surfaces, as shown in FIG. 7, can be used.
As the metal foil 252, for example, an aluminum foil can be used.
As the polymer film 254, e.g., a polypropylene film can be used. As
a material for the outer polymer film 254, a polymer having a high
melting point is preferable and, for example, polyethylene
terephthalate (PET) and polyamide are more preferable. As a
material for the inner polymer film 254, e.g., polyethylene and
polypropylene are preferable.
[0199] The leads 260, 262 are formed of a conductive material such
as aluminum. Ends of the leads 260, 262 are connected to an end of
the positive electrode collector 212 and an end of the negative
electrode collector 222, respectively. The other ends of the leads
260, 262 extend outside the case 250.
[0200] Next, a method for manufacturing the lithium ion secondary
battery 200 will be described.
[0201] First, the positive electrode 210 and the negative electrode
220 are prepared as follows. An active material particle, a binder
and a necessary amount of conductive auxiliary are added to a
solvent such as N-methyl-2-pyrrolidone or N,N-dimethylformamide to
prepare a slurry. The slurry was applied to the surface of the
collectors 212, 222 and dried to obtain the positive electrode
active-material containing layer 214, and the negative electrode
active-material containing layer 224. Next, a silicone resin and a
binder are added to a solvent such as N-methyl-2-pyrrolidone or
N,N-dimethylformamide to obtain a slurry. The slurry is applied to
the surfaces of the positive electrode active-material containing
layer 214 and the negative electrode active-material containing
layer 224 and dried to obtain the protecting layers 216, 226. In
this manner, the positive electrode 210 and the negative electrode
220 can be obtained.
[0202] Furthermore, other than the aforementioned positive
electrode 210 and negative electrode 220, an electrolytic solution,
a separator 218, case 250 and leads 260, 262 are prepared.
[0203] The electrolytic solution is allowed to contain within the
positive electrode active-material containing layer 214, the
positive electrode protecting layer 216, the separator 218, the
negative electrode active-material containing layer 224 and the
negative electrode protecting layer 226. The electrolytic solution
is not particularly limited and, for example, an electrolytic
solution (aqueous electrolytic solution, an electrolytic solution
using an organic solvent) containing a lithium salt can be used.
Note that if an aqueous electrolytic solution is electrochemically
decomposed at a low voltage, a withstand voltage during electrical
charge is low and limited. For this reason, an electrolytic
solution (non-aqueous electrolytic solution) using an organic
solvent is preferably used. As the electrolytic solution an
electrolytic solution dissolving a lithium salt in a non-aqueous
solvent (an organic solvent) is preferably used. Examples of the
lithium salt include salts such as LiPF.sub.6, LiClO.sub.4,
LiBF.sub.4, LiAsF.sub.6, LiCF.sub.3SO.sub.3, LiCF.sub.3,
CF.sub.2SO.sub.3, LiC(CF.sub.3SO.sub.2).sub.3,
LiN(CF.sub.3SO.sub.2).sub.2, LiN(CF.sub.3CF.sub.2SO.sub.2).sub.2,
LiN(CF.sub.3SO.sub.2)(C.sub.4F.sub.9SO.sub.2),
LiN(CF.sub.3CF.sub.2CO).sub.2 and LiBOB. Note that, these salts may
be used singly or in combination with two or more types in an
arbitrary ratio.
[0204] Furthermore, as the organic solvent, for example, propylene
carbonate, ethylene carbonate and diethyl carbonate, etc. are
preferred. These may be used singly or in combination with two or
more types in an arbitrary ratio.
[0205] Note that, the electrolytic solution may be not only a
liquid-state electrolyte but also a gel-state electrolyte, which is
obtained by adding a gelatinizing agent. Furthermore, a solid
electrolyte (a solid polymer electrolyte or an electrolyte made of
an ion conductive inorganic material) may be contained in place of
an electrolytic solution.
[0206] Subsequently, according to a known method, the leads 260,
262 are welded to the positive electrode collector 212 and the
negative electrode collector 222, respectively. A construct having
the separator 218 sandwiched by the positive electrode protecting
layer 216 of the positive electrode 210 and the negative electrode
protecting layer 226 of the negative electrode 220 is inserted in
the case 250 together with an electrolytic solution and the inlet
of the case 250 is sealed. In this manner mentioned above, the
lithium ion secondary battery 200 can be obtained.
[0207] In the embodiment, it is possible to suppress rate
characteristics from deteriorating. In addition, it is possible to
sufficiently suppress short circuit even at high temperatures. The
reason is unknown; however, the present inventors consider as
follows. In the embodiment, since the protecting layers 216, 226
contain a silicone resin particle as mentioned above, even if the
thicknesses of the protecting layers 216, 226 are reduced to, for
example, about 1 to 6 fold as small as the particle size of the
silicone resin particle, the protecting layers 216, 226 can be
obtained with relatively uniform thickness. Therefore, it is not
necessary to sufficiently reduce the size of the particles
constituting the protecting layers 216, 226 compared to the
thickness of the protecting layers, in order to obtain the
protecting layers 216, 226 having a uniform thickness. Therefore,
as the particle constituting the protecting layers 216, 226, a
particle having a relatively large size close to the thicknesses of
the protecting layers 216, 226 can be used. By virtue of this, the
interspace between particles is widened and ions in an electrolytic
solution can easily migrate between the particles. Therefore, it is
considered that rate characteristics is suppressed from
deteriorating.
[0208] Furthermore, in the embodiment, the silicone resin particle
has a siloxane bond. Therefore, even in a high temperature (for
example, 400.degree. C.), the protecting layers 216, 226 can be
suppressed from melting. By virtue of this, the heat resistance of
the protecting layers 216, 226 is improved and the short circuit
prevention function of protecting layers 216, 226 can be maintained
during a high-temperature operation time, such as an overcharge
test. Furthermore, a process for manufacturing a lithium ion
secondary battery, even if a separator is broken, short circuit
between the positive electrode 210 and the negative electrode 220
can be suppressed by arranging the insulating protecting layers
216, 226.
[0209] The present invention is not limited to the embodiments
above and can be variously modified. For example, a protecting
layer may be provided only one of the positive electrode 210 and
the negative electrode 220.
Examples
[0210] The present invention will be more specifically described
based on Examples and Comparative Examples below; however, the
present invention is not limited to the following Examples.
Example 1-1
[0211] (Preparation of Negative Electrode)
[0212] First, 1.5 parts by mass of sodium carboxymethylcellulose
(trade name: Cellogen WS-C manufactured by Dai-ichi Kogyo Seiyaku
Co., Ltd.) was dissolved in pure water (purified through an ion
exchange membrane and distillated). To the dissolution solution,
93.5 parts by mass of natural graphite (trade name: HG-702,
manufactured by Hitachi Chemical Co., Ltd.), 2.0 parts by mass of
acetylene black (trade name: Denka, Black, manufactured by Denki
Kagaku Kogyo K.K.) and 3.0 parts by mass of styrene-butadiene
rubber (trade name: SN-307R manufactured by Nippon A&L Inc.)
were added, mixed and dispersed by a planetary mixer to obtain a
slurry-state coating solution for forming a negative electrode
active-material layer. The coating solution was applied to both
surfaces of a copper foil having a thickness of 15 .mu.m by a
doctor blade method, dried and pressed by a calender roll to form a
negative electrode active-material layer having a thickness (one
surface) of 80 .mu.m.
[0213] Next, as an organic particle, 95.5 parts by mass of a
poly(methyl methacrylate) particle having a crosslinked structure
and having an average particle size (D50) of 2.0 .mu.m and a ratio
of major-axis length/minor-axis length of 1.03 (which was
constituted of beads obtained by classification from crosslinked
acrylic beads (trade name: Art Pearl series, manufactured by Negami
Chemical Industrial Co., Ltd.), 3.0 parts by mass of
styrene-butadiene rubber (trade name: SN-307R manufactured by
Nippon A&L Inc.) and 1.5 parts by mass of sodium
carboxymethylcellulose (trade name: Cellogen WS-C manufactured by
Dai-ichi Kogyo Seiyaku Co., Ltd.) were mixed and dissolved in pure
water (purified through an ion exchange membrane and distillated)
to obtain a slurry-state coating solution for forming a protecting
layer. The coating solution was applied to each negative electrode
active-material layer by a doctor blade method and dried to obtain
a protecting layer having a thickness of 2.0 .mu.m (one surface).
By virtue of this, a negative electrode having a negative electrode
active-material layer and a protecting layer formed on both
surfaces of a collector (two-surface coated negative electrode) was
obtained.
[0214] (Preparation of Positive Electrode)
[0215] First, 44.5 parts by mass of lithium nickel-cobalt manganate
(trade name: NCM-01ST-5, manufactured by Toda Kogyo Corp.), 44.5
parts by mass of lithium-manganese spinel (trade name: HPM-6050
manufactured by Toda Kogyo Corp.), 3.0 parts by mass of acetylene
black (trade name: Denka Black, manufactured by Denki Kagaku Kogyo
K.K.), 3.0 parts by mass of graphite (trade name: KS-6,
manufactured by Timcal Ltd.) and 5.0 parts by mass of polyvinyldene
fluoride (PVDF) (trade name: KYNAR-761, manufactured by Arkema
Inc.) were mixed and dissolved in N-methylpyrrolidone (NMP) to
obtain a slurry-state coating solution for forming a positive
electrode active-material layer. The coating solution was applied
to both surfaces of an aluminum foil having a thickness of 20 .mu.m
by a doctor blade method, dried and pressed by a calender roll to
form a positive electrode active-material layer having a thickness
(one surface) of 95 .mu.m. In this manner, a positive electrode
(two-surface coated positive electrode) having a positive electrode
active-material layer formed on both surfaces of a collector was
obtained.
[0216] (Preparation of Electrolytic Solution)
[0217] First, 20 parts by volume of propylene carbonate (PC), 10
parts by volume of ethylene carbonate (EC) and 70 parts by volume
of diethyl carbonate were mixed to obtain a solvent mixture. In the
solvent mixture, lithium hexafluorophosphate (LiPF.sub.6) was
dissolved so as to obtain a concentration of 1.5 moldm.sup.-3 to
obtain an electrolytic solution.
[0218] (Preparation of Lithium Ion Secondary Battery)
[0219] A two-surface coated negative electrode (dimension of 31.0
mm.times.41.5 mm) in a shape having a tongue portion was obtained
by stamping. A two-surface coated positive electrode (dimension of
30.5 mm.times.41.0 mm) in a shape having a tongue portion was
obtained by stamping. Furthermore, a separator formed of
polyethylene and having a dimension of 32.0 mm.times.43.0 mm was
prepared. Six sheets of two-surface coated negative electrodes and
five sheets of two-surface coated positive electrodes were
alternately laminated with the separator interposed between them to
form a laminate having the laminate structure: two-surface coated
negative electrode/separator/two-surface coated positive
electrode/separator/two-surface coated negative
electrode/separator/two-surface coated positive
electrode/separator/two-surface coated negative
electrode/separator/two-surface coated positive
electrode/separator/two-surface coated negative
electrode/separator/two-surface coated positive
electrode/separator/two-surface coated negative
electrode/separator/two-surface coated positive
electrode/separator/two-surface coated negative electrode. The
resultant laminate structure was housed in an aluminum laminate
film and an electrolytic solution was injected and sealed under
vacuum. In this manner, a lithium ion secondary battery was
prepared which had the same structure as that shown in FIG. 5 and
FIG. 6 except the number of two-surface coated negative electrodes
and the number of two-surface coated positive electrodes
laminated.
Example 1-2
[0220] A lithium ion secondary battery was prepared in the same
manner as in Example 1-1 except that, as the organic particle of
the protecting layer, use was made of a poly(methyl methacrylate)
particle having a crosslinked structure and having an average
particle size (D50) of 0.5 .mu.m and a ratio of major-axis
length/minor-axis length of 1.03 (which was constituted of beads
obtained by classification from crosslinked acrylic beads (trade
name: Art Pearl series), manufactured by Negami Chemical Industrial
Co., Ltd).
Example 1-3
[0221] A lithium ion secondary battery was prepared in the same
manner as in Example 1-1 except that, as the organic particle of
the protecting layer, use was made of a poly(methyl methacrylate)
particle having a crosslinked structure and having an average
particle size (D50) of 4.0 .mu.m and a ratio of major-axis
length/minor-axis length of 1.03 (which was constituted of beads
obtained by classification from crosslinked acrylic beads (trade
name: Art Pearl series), manufactured by Negami Chemical Industrial
Co., Ltd.) and further the thickness of the protecting layer was
set to 4.0 .mu.m.
Example 1-4
[0222] A lithium ion secondary battery was prepared in the same
manner as in Example 1-1 except that, as the organic particle of
the protecting layer, use was made of a poly(methyl methacrylate)
particle having a crosslinked structure and having an average
particle size (D50) of 2.0 .mu.m and a ratio of major-axis
length/minor-axis length of 1.30 (which was constituted of beads
obtained by classification from crosslinked acrylic beads (trade
name: Art Pearl series), manufactured by Negami Chemical Industrial
Co., Ltd).
Example 1-5
[0223] A lithium ion secondary battery was prepared in the same
manner as in Example 1-1 except that, as the organic particle of
the protecting layer, use was made of a poly(methyl methacrylate)
particle having a crosslinked structure and having an average
particle size (D50) of 2.0 .mu.m and a ratio of major-axis
length/minor-axis length of 2.00 (which was constituted of beads
obtained by classification from crosslinked acrylic beads (trade
name: Art Pearl series), manufactured by Negami Chemical Industrial
Co., Ltd).
Comparative Example 1-1
[0224] A lithium ion secondary battery was prepared in the same
manner as in Example 1-1 except that, as the organic particle of
the protecting layer, use was made of a non-crosslinked poly(methyl
methacrylate) powder (an average particle size (D50): 2.0 .mu.m,
ratio of major-axis length/minor-axis length of 1.03).
Comparative Example 1-2
[0225] A lithium ion secondary battery was prepared in the same
manner as in Example 1-1 except that, as the organic particle of
the protecting layer, use was made of a polyethylene (PE) particle
having an average particle size (D50) of 2.0 .mu.m and a ratio of
major-axis length/minor-axis length of 1.03 (which was constituted
of beads obtained by classification from Flowsen (trade name)
manufactured by Sumitomo Seika Chemicals Co., Ltd).
Comparative Example 1-3
[0226] A lithium ion secondary battery was prepared in the same
manner as in Example 1-1 except that, as the organic particle of
the protecting layer, use was made of a polytetrafluoroethylene
(PTFE) particle having an average particle size (D50) of 2.0 .mu.m
and a ratio of major-axis length/minor-axis length of 1.03 (which
was constituted of beads by classification from SST series (trade
name) manufactured by SHAMROCK TECHNOLOGIES).
Comparative Example 1-4
[0227] A lithium ion secondary battery was prepared in the same
manner as in Example 1-1 except that, as the organic particle of
the protecting layer, use was made of an inorganic particle, more
specifically, an alumina particle having an average particle size
(D50) of 0.20 .mu.m (which was constituted of beads obtained by
classification from AKP (trade name) manufactured by Sumitomo
Chemical Co., Ltd).
Comparative Example 1-5
[0228] A lithium ion secondary battery was prepared in the same
manner as in Example 1-1 except that, as the organic particle of
the protecting layer, use was made of an inorganic particle, more
specifically, an alumina particle having an average particle size
(D50) of 2.0 .mu.m (which was constituted of beads obtained by
classification from AL (trade name) manufactured by Sumitomo
Chemical Co., Ltd). Note that the thickness of the protecting layer
varied between 2.0 to 4.0 .mu.m.
Comparative Example 1-6
[0229] A lithium ion secondary battery was prepared in the same
manner as in Example 1-1 except that, as the organic particle of
the protecting layer, use was made of a poly(methyl methacrylate)
particle having a crosslinked structure and having an average
particle size (D50) of 0.3 .mu.m and a ratio of major-axis
length/minor-axis length of 1.03 (which was constituted of beads
obtained by classification from crosslinked acrylic beads (trade
name: Art Pearl series), manufactured by Negami Chemical Industrial
Co., Ltd).
Comparative Example 1-7
[0230] A lithium ion secondary battery was prepared in the same
manner as in Example 1-1 except that, as the organic particle of
the protecting layer, use was made of a poly(methyl methacrylate)
particle having a crosslinked structure and having an average
particle size (D50) of 6.0 .mu.m and a ratio of major-axis
length/minor-axis length of 1.03 (which was constituted of beads
obtained by classification from crosslinked acrylic beads (trade
name: Art Pearl series), manufactured by Negami Chemical Industrial
Co., Ltd.) and further the thickness of the protecting layer was
set to 6.0 .mu.m.
[0231] <Measurement of Rate Characteristics>
[0232] With respect to the lithium ion secondary batteries obtained
in Examples and Comparative Examples, discharged capacity was
measured at 1 C (the amount of current discharged for one hour when
constant-current discharge is performed at 25.degree. C.) and at 5
C (the amount of current discharged for 0.2 hours when
constant-current discharge is performed at 25.degree. C.). The
ratio (%) of discharged capacity obtained at 5 C to the discharged
capacity (regarded as 100%) at 1 C was obtained as rate
characteristics. The results are shown in Table 1.
[0233] <Charge-Discharge Cycle Test>
[0234] The lithium ion secondary batteries obtained in Examples and
Comparative Examples were charged in a CCCV charge operation (4.2
V) at a rate of 1 C. Thereafter, constant-current discharge was
performed to 2.5 V at a rate of 1 C. The charge-discharge operation
was performed under an environment temperature of 45.degree. C. The
charge-discharge operation (regarded it as a single cycle) was
repeated 500 times. The difference in thickness between the lithium
ion secondary battery after 500 cycles and initial thickness
thereof (thickness after 500 cycles--initial thickness) was
obtained and regarded as swelling of the cell. The results are
shown in Table 1. Note that the initial thickness of a lithium ion
secondary battery varies depending upon the thickness of a
protecting layer but is about 2.80 mm. The smaller the swelling of
the cell, the more dendrite growth is suppressed, meaning that the
cell is excellent in charge-discharge cycle characteristics and
safety.
TABLE-US-00001 TABLE 1 Ratio of Average size major-axis [.mu.m] of
Thickness Average size length/minor- inorganic of Rate [.mu.m] and
axis length of particle protecting characteristics Swelling of
material for organic (alumina) layer 5 C/1 C cell organic particle
particle [.mu.m] [.mu.m] [%] [mm] Ex. 1-1 2.0 1.03 -- 2.0 48 No
swelling Crosslinked of cell PMMA (less than 0.1) Ex. 1-2 0.5 1.03
-- 2.0 47 No swelling Crosslinked of cell PMMA (less than 0.1) Ex.
1-3 4.0 1.03 -- 4.0 42 No swelling Crosslinked of cell PMMA (less
than 0.1) Ex. 1-4 2.0 1.30 -- 2.0 44 No swelling Crosslinked of
cell PMMA (less than 0.1) Ex. 1-5 2.0 2.00 -- 2.0 41 No swelling
Crosslinked of cell PMMA (less than 0.1) Com. Ex. 1-1 2.0 1.03 --
2.0 44 0.5 Non- crosslinked PMMA Com. Ex. 1-2 2.0 1.03 -- 2.0 48
1.2 PE Com. Ex. 1-3 2.0 1.03 -- 2.0 46 1.1 PTFE Com. Ex. 1-4 -- --
0.20 2.0 45 0.3 Com. Ex. 1-5 -- -- 2.0 2.0~4.0 39 No swelling of
cell (less than 0.1) Com. Ex. 1-6 0.3 1.03 -- 2.0 46 0.5
Crosslinked PMMA Com. Ex. 1-7 6.0 1.03 -- 6.0 28 No swelling
Crosslinked of cell PMMA (less than 0.1)
Example 2-1
[0235] (Preparation of Negative Electrode)
[0236] First, 1.5 parts by mass of sodium carboxymethylcellulose
(trade name: Cellogen WS-C, manufactured by Dai-ichi Kogyo Seiyaku
Co., Ltd.) was dissolved in pure water (purified through an ion
exchange membrane and distillated). To the dissolved solution, 93.5
parts by mass of natural graphite (Trade name: HG-702, manufactured
by Hitachi Chemical Co., Ltd.), 2.0 parts by mass of acetylene
black (Trade name: Denka Black, manufactured by Denki Kagaku Kogyo
K.K.) and 3.0 parts by mass of styrene-butadiene rubber (trade
name: SN-307R, manufactured by Nippon A&L Inc.) were added,
mixed and dispersed by a planetary mixer to obtain a slurry-state
coating solution for forming a negative electrode active-material
layer. The coating solution was applied to both surfaces of a
copper foil having a thickness of 15 .mu.m by a doctor blade
method, dried and pressed by a calender roll to form a negative
electrode active-material layer having a thickness (one surface) of
80 .mu.m.
[0237] Next, 31.8 parts by mass of a polyethylene particle having
an average particle size (D50) of 0.50 .mu.m (which was constituted
of beads obtained by classification from Flow beads HE series
(trade name) manufactured by Sumitomo Seika Chemicals Co., Ltd.) as
an organic particle, 63.7 parts by mass of an alumina particle
(trade name: HIT series, manufactured by Sumitomo Chemical Co.,
Ltd., an average particle size (D50): 0.20 .mu.m) as an inorganic
particle, 3.0 parts by mass of styrene-butadiene rubber (trade
name: SN-307R manufactured by Nippon A&L Inc.) and 1.5 parts by
mass of sodium carboxymethylcellulose (trade name: Cellogen WS-C,
manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd.) were mixed and
dissolved in pure water (purified through an ion exchange membrane
and distillated) to obtain a slurry-state coating solution for
forming a protecting layer. The coating solution was applied to
each negative electrode active-material layer by a doctor blade
method and dried to obtain a protecting layer having a thickness
(one surface) of 2.0 .mu.m. In this manner, a negative electrode
(two-surface coated negative electrode) having a negative electrode
active-material layer and a protecting layer formed on both
surfaces of a collector was obtained.
[0238] (Preparation of Positive Electrode)
[0239] First, 44.5 parts by mass of lithium nickel-cobalt manganate
(trade name: NCM-01ST-5, manufactured by Toda Kogyo Corp), 44.5
parts by mass of lithium-manganese spinel (trade name: HPM-6050
manufactured by Toda Kogyo Corp.), 3.0 parts by mass of acetylene
black (trade name: Denka Black, manufactured by Denki Kagaku Kogyo
K.K.), 3.0 parts by mass of graphite (trade name: KS-6,
manufactured by Timcal Ltd.) and 5.0 parts by mass of polyvinyldene
fluoride (PVDF) (trade name: KYNAR-761, manufactured by Arkema
Inc.) were mixed and dissolved in N-methylpyrrolidone (NMP) to
obtain a slurry-state coating solution for forming a positive
electrode active-material layer. The coating solution was applied
to both surfaces of an aluminum foil having a thickness of 20 .mu.m
by a doctor blade method, dried and pressed by a calender roll to
form a positive electrode active-material layer having a thickness
(one surface) of 95 .mu.m. In this manner, a positive electrode
(two-surface coated positive electrode) having a positive electrode
active-material layer formed on both surfaces of a collector was
obtained.
[0240] (Preparation of Electrolytic Solution)
[0241] First, 20 parts by volume of propylene carbonate (PC), 10
parts by volume of ethylene carbonate (EC) and 70 parts by volume
of diethyl carbonate were mixed to obtain a solvent mixture. In the
solvent mixture, lithium hexafluorophosphate (LiPF.sub.6) was
dissolved so as to obtain a concentration of 1.5 moldm.sup.-3 to
obtain an electrolytic solution.
[0242] (Preparation of Lithium Ion Secondary Battery)
[0243] A two-surface coated negative electrode (dimension of 31.0
mm.times.41.5 mm) in a shape having a tongue portion was obtained
by stamping. A two-surface coated positive electrode (dimension of
30.5 mm.times.41.0 mm) in a shape having a tongue portion was
obtained by stamping. Furthermore, a separator formed of
polyethylene and having a dimension of 32.0 mm.times.43.0 mm was
prepared. Six sheets of two-surface coated negative electrodes and
five sheets of two-surface coated positive electrodes were
alternately laminated with the separator interposed between them to
form a laminate having the laminate structure: two-surface coated
negative electrode/separator/two-surface coated positive
electrode/separator/two-surface coated negative
electrode/separator/two-surface coated positive
electrode/separator/two-surface coated negative
electrode/separator/two-surface coated positive
electrode/separator/two-surface coated negative
electrode/separator/two-surface coated positive
electrode/separator/two-surface coated negative
electrode/separator/two-surface coated positive
electrode/separator/two-surface coated negative electrode. The
resultant laminate structure was housed in an aluminum laminate
film and an electrolytic solution was injected and sealed under
vacuum. In this manner, a lithium ion secondary battery was
prepared, which had the same structure as that shown in FIG. 5 and
FIG. 6 except the number of two-surface coated negative electrodes
and the number of two-surface coated positive electrodes
laminated.
Example 2-2
[0244] A lithium ion secondary battery was prepared in the same
manner as in Example 2-1 except that, as the organic particle of
the protecting layer, use was made of a polypropylene particle
having an average particle size (D50) of 0.5 .mu.m obtained by
classification of a polypropylene particle (trade name: [TRL-PP-101
series], manufactured by Trial Corporation).
Example 2-3
[0245] A lithium ion secondary battery was prepared in the same
manner as in Example 2-1 except that, as the inorganic particle of
the protecting layer, use was made of an alumina particle having an
average particle size (D50):0.50 .mu.m (trade name: AU series
manufactured by Sumitomo Chemical Co., Ltd).
Example 2-4
[0246] A lithium ion secondary battery was prepared in the same
manner as in Example 2-1 except that, as the inorganic particle of
the protecting layer, use was made of a silica particle having an
average particle size (D50) of 0.2 .mu.m and obtained by
classification from SICASTAR (trade name), plain type, manufactured
by Corefront Corporation).
Example 2-5
[0247] A lithium ion secondary battery was prepared in the same
manner as in Example 2-1 except that the thickness of the
protecting layer was set to 3.5
Example 2-6
[0248] A lithium ion secondary battery was prepared in the same
manner as in Example 2-1 except that the thickness of the
protecting layer was set to 4.0 .mu.m.
Example 2-7
[0249] A lithium ion secondary battery was prepared in the same
manner as in Example 2-1 except that the thickness of the
protecting layer was set to 4.5 .mu.m.
Example 2-8
[0250] A lithium ion secondary battery was prepared in the same
manner as in Example 2-1 except that the thickness of the
protecting layer was set to 6.0 .mu.m.
Example 2-9
[0251] A lithium ion secondary battery was prepared in the same
manner as in Example 2-1 except that the thickness of the
protecting layer was set to 1.0 .mu.m.
Example 2-10
[0252] A lithium ion secondary battery was prepared in the same
manner as in Example 2-1 except that the thickness of the
protecting layer was set to 0.5 .mu.m.
Example 2-11
[0253] A lithium ion secondary battery was prepared in the same
manner as in Example 2-1 except that the average particle size
(D50) of the polyethylene particle classified was set to 0.10 .mu.m
and the thickness of the protecting layer was set to 0.25
.mu.m.
Example 2-12
[0254] A lithium ion secondary battery was prepared in the same
manner as in Example 2-1 except that, in the protecting layer, the
content of the organic particle was set to 47.7 parts by mass, and
the content of the inorganic particle was set to 47.8 parts by
mass.
Example 2-13
[0255] A lithium ion secondary battery was prepared in the same
manner as in Example 2-1 except that, in the protecting layer, the
content of the organic particle was set to 41.5 parts by mass, and
the content of the inorganic particle was set to 53.9 parts by
mass.
Example 2-14
[0256] A lithium ion secondary battery was prepared in the same
manner as in Example 2-1 except that, in the protecting layer, the
content of the organic particle was set to 38.2 parts by mass, and
the content of the inorganic particle was set to 57.3 parts by
mass.
Example 2-15
[0257] A lithium ion secondary battery was prepared in the same
manner as in Example 2-1 except that, in the protecting layer, the
content of the organic particle was set to 23.9 parts by mass, and
the content of the inorganic particle was set to 71.6 parts by
mass.
Example 2-16
[0258] A lithium ion secondary battery was prepared in the same
manner as in Example 2-1 except that, in the protecting layer, the
content of the organic particle was set to 22.7 parts by mass, and
the content of the inorganic particle was set to 72.8 parts by
mass.
Example 2-17
[0259] A lithium ion secondary battery was prepared in the same
manner as in Example 2-1 except that, in the protecting layer, the
content of the organic particle was set to 19.1 parts by mass, and
the content of the inorganic particle was set to 76.4 parts by
mass.
Example 2-18
[0260] A lithium ion secondary battery was prepared in the same
manner as in Example 2-1 except that the average particle size
(D50) of the alumina particle classified was set to 1.00 .mu.m.
Example 2-19
[0261] A lithium ion secondary battery was prepared in the same
manner as in Example 2-1 except that the average particle size
(D50) of the alumina particle classified was set to 0.16 .mu.m.
Example 2-20
[0262] A lithium ion secondary battery was prepared in the same
manner as in Example 2-1 except that the average particle size
(D50) of the alumina particle classified was set to 0.13 .mu.m.
Example 2-21
[0263] A lithium ion secondary battery was prepared in the same
manner as in Example 2-1 except that, as the organic particle of
the protecting layer, a polyethylene particle having an average
particle size (D50) of 0.10 .mu.m and obtained by classification
from Flow beads HE series (trade name, manufactured by Sumitomo
Seika Chemicals Co., Ltd.) was used and, as the inorganic particle,
an alumina particle (trade name: HIT series, an average particle
size (D50): 0.10 .mu.m, manufactured by Sumitomo Chemical Co.,
Ltd.) was used.
Example 2-22
[0264] A lithium ion secondary battery was prepared in the same
manner as in Example 2-1 except that, as the organic particle of
the protecting layer, a polyethylene particle having an average
particle size (D50) of 4.0 .mu.m obtained by classification from
Flow beads HE series, (trade name manufactured by Sumitomo Seika
Chemicals Co., Ltd.) was used and, as the inorganic particle, an
alumina particle (trade name: AL series, an average particle size
(D50): 4.0 .mu.m, manufactured by Sumitomo Chemical Co., Ltd.) was
used.
Comparative Example 2-1
[0265] A lithium ion secondary battery was prepared in the same
manner as in Example 2-1 except that the protecting layer was not
provided.
Comparative Example 2-2
[0266] A lithium ion secondary battery was prepared in the same
manner as in Example 2-1 except that, in the protecting layer, the
organic particle was not used and the content of the inorganic
particle was set to 95.5 parts by mass.
Comparative Example 2-3
[0267] A lithium ion secondary battery was prepared in the same
manner as in Example 2-1 except that, in the protecting layer, the
inorganic particle was not used and the content of the organic
particle was set to 95.5 parts by mass.
Comparative Example 2-4
[0268] A lithium ion secondary battery was prepared in the same
manner as in Example 2-1 except that, in the protecting layer, the
content of the organic particle was set to 63.7 parts by mass and
the content of the inorganic particle was set to 31.8 parts by
mass.
Comparative Example 2-5
[0269] A lithium ion secondary battery was prepared in the same
manner as in Example 2-1 except that, in the protecting layer, the
content of the organic particle was set to 15.9 parts by mass and
the content of the inorganic particle was set to 79.6 parts by
mass.
Comparative Example 2-6
[0270] A lithium ion secondary battery was prepared in the same
manner as in Example 2-1 except that, as the organic particle of
the protecting layer, a polyethylene particle having an average
particle size (D50) of 0.05 .mu.m and obtained by classification
from Flow beads HE series (trade name, manufactured by Sumitomo
Seika Chemicals Co., Ltd.) was used and, as the inorganic particle,
an alumina particle (trade name: HIT series, an average particle
size (D50): 0.05 .mu.m, manufactured by Sumitomo Chemical Co.,
Ltd.) was used.
Comparative Example 2-7
[0271] A lithium ion secondary battery was prepared in the same
manner as in Example 2-1 except that, as the organic particle of
the protecting layer, a polyethylene particle having an average
particle size (D50): 6.0 .mu.m and obtained by classification from
Flow beads HE series, (trade name, manufactured by Sumitomo Seika
Chemicals Co., Ltd.) was used, and as the inorganic particle, an
alumina particle (trade name: AL series, an average particle size
(D50): 6.0 .mu.m, manufactured by Sumitomo Chemical Co., Ltd.) was
used and the thickness of the protecting layer was set to 7.0
.mu.m.
Comparative Example 2-8
[0272] A lithium ion secondary battery was prepared in the same
manner as in Example 2-1 except that, as the organic particle of
the protecting layer, a polytetrafluoroethylene (PTFE) particle
having a an average particle size (D50) of 0.50 .mu.m and obtained
by classification from SST series (trade name, manufactured by
SHAMROCK TECHNOLOGIES) was used.
[0273] <Measurement of Impedance>
[0274] The lithium ion secondary batteries obtained in Examples and
Comparative Examples were measured for impedance (m.OMEGA.) at an
alternate current of 1 kHz by an impedance analyzer (SI 1287, SI
1260) manufactured by Toyo Corporation. Note that, impedance was
measured at an environment temperature of 25.degree. C. and a
relative humidity of 60%. The results are shown in Tables 2 to
4.
[0275] <Measurement of rate Characteristics>
[0276] With respect to the lithium ion secondary batteries obtained
in Examples and Comparative Examples, discharged capacity was
measured at 1 C (the amount of current discharged for one hour when
constant-current discharge is performed at 25.degree. C.) and at 5
C (the amount of current discharged for 0.2 hours when
constant-current discharge is performed at 25.degree. C.) and the
ratio (%) of discharged capacity obtained at 5 C to the discharged
capacity (regarded as 100%) at 1 C was obtained as rate
characteristics. The results are shown in Tables 2 to 4.
[0277] <Overcharge Test>
[0278] The lithium ion secondary batteries obtained in Examples and
Comparative Examples were previously subjected to a
charge-discharge process performed at a rate as low as 0.05 C at
25.degree. C. and then subjected to a CC charge operation performed
at 3 C until 10 V, and thereafter, CV charge was maintained until
the temperature of the cell decreased. The maximum surface
temperature of the cell surface was obtained based on temperature
measurement and a change in shape of cells was observed. The
results of them were used as the evaluation results of the
overcharge test. The results are shown in Tables 2 to 4. Note that,
in the overcharge test, the lithium ion secondary batteries that
caused no cell burst can be evaluated that the protecting layer has
a sufficient shutdown function and safety during a heat-up time is
excellent. Furthermore, in the batteries that caused no burst, the
lower the cell temperature, the more an increase of inner
temperature of the cell was suppressed, meaning that the battery is
high in safety. The cell temperature is preferably 80.degree. C. or
less.
[0279] <Temperature Raising Test>
[0280] The lithium ion secondary batteries obtained in Examples and
Comparative Examples were previously subjected to a
charge-discharge process performed at a rate as low as 0.05 C at
25.degree. C. and then, subjected to a charge operation performed
at 1 C until 4.2 V. Thereafter, the thickness of the cell (before
storage) was measured. After that, the lithium ion secondary
battery was loaded in an oven and the temperature was increased at
a temperature raising rate of 5.degree. C/minute until the
temperature reached 150.degree. C. and stored at 150.degree. C. for
one hour. Thereafter, the state of the cell was observed. The cell
that did not burst was measured for thickness (after storage). The
difference in thickness before and after storage was obtained as a
degree of swelling (mm). The results are shown in Tables 2 to 4.
Note that, the lithium ion secondary batteries that did not burst
in the temperature raising test can be evaluated that shrinkage of
the protecting layer is sufficiently suppressed and safety during a
heat-up time is excellent.
TABLE-US-00002 TABLE 2 Average Average Overcharge size [.mu.m] size
[.mu.m] Content Thickness test Temperature and and ratio of Rate 3
C-10 V raising test material for material for (organic protecting
characteristics (state of cell 25~150.degree. C. organic inorganic
particle:inorganic layer Impedance 5 C/1 C & cell (state of
cell & particle particle particle) [.mu.m] [m.OMEGA.] [%]
temperature) swelling) Ex. 2-1 0.50 0.20 1:2 2.0 74 48 No burst No
burst PE Alumina (68.degree. C.) (degree of swelling: 0.5 mm or
less) Ex. 2-2 0.50 0.20 1:2 2.0 74 47 No burst No burst PP Alumina
(68.degree. C.) (degree of swelling: 0.5 mm or less) Ex. 2-3 0.50
0.50 1:2 2.0 77 47 No burst No burst PE Alumina (65.degree. C.)
(degree of swelling: 0.5 mm or less) Ex. 2-4 0.50 0.20 1:2 2.0 76
47 No burst No burst PE silica (66.degree. C.) (degree of swelling:
0.5 mm or less) Ex. 2-5 0.50 0.20 1:2 3.5 78 45 No burst No burst
PE Alumina (64.degree. C.) (degree of swelling: 0.5 mm or less) Ex.
2-6 0.50 0.20 1:2 4.0 81 44 No burst No burst PE Alumina
(63.degree. C.) (degree of swelling: 0.5 mm or less) Ex. 2-7 0.50
0.20 1:2 4.5 85 42 No burst No burst PE Alumina (61.degree. C.)
(degree of swelling: 0.5 mm or less) Ex. 2-8 0.50 0.20 1:2 6.0 88
40 No burst No burst PE Alumina (61.degree. C.) (degree of
swelling: 0.5 mm or less) Ex. 2-9 0.50 0.20 1:2 1.0 72 46 No burst
No burst PE Alumina (73.degree. C.) (degree of swelling: 0.5 mm or
less) Ex. 2-10 0.50 0.20 1:2 0.5 70 50 No burst No burst PE Alumina
(77.degree. C.) (degree of swelling: 0.5 mm or less) Ex. 2-11 0.10
0.20 1:2 0.25 67 53 No burst No burst PE Alumina (80.degree. C.)
(degree of swelling: 0.5 mm or less)
TABLE-US-00003 TABLE 3 Average Average Overcharge size [.mu.m] size
[.mu.m] Content Thickness test Temperature and and ratio of Rate 3
C-10 V raising test material for material for (organic protecting
characteristics (state of cell 25~150.degree. C. organic inorganic
particle:inorganic layer Impedance 5 C/1 C & cell (state of
cell & particle particle particle) [.mu.m] [m.OMEGA.] [%]
temperature) swelling) Ex. 2-12 0.50 0.20 1:1 2.0 74 49 No burst No
burst PE Alumina (65.degree. C.) (Degree of swelling: 1.6 mm) Ex.
2-13 0.50 0.20 1:1.3 2.0 74 48 No burst No burst PE Alumina
(65.degree. C.) (Degree of swelling: 1.1 mm) Ex. 2-14 0.50 0.20
1:1.5 2.0 74 47 No burst No burst PE Alumina (65.degree. C.)
(Degree of swelling: 0.8 mm) Ex. 2-15 0.50 0.20 1:3 2.0 74 47 No
burst No burst PE Alumina (71.degree. C.) (Degree of swelling: 0.5
mm or less) Ex. 2-16 0.50 0.20 1:3.2 2.0 74 47 No burst No burst PE
Alumina (78.degree. C.) (Degree of swelling: 0.5 mm or less) Ex.
2-17 0.50 0.20 1:4 2.0 75 45 No burst No burst PE Alumina
(79.degree. C.) (Degree of swelling: 0.5 mm or less) Ex. 2-18 0.50
1.00 1:2 2.0 89 47 No burst No burst PE Alumina (68.degree. C.)
(Degree of swelling: 0.5 mm or less) Ex. 2-19 0.50 0.16 1:2 2.0 70
46 No burst No burst PE Alumina (75.degree. C.) (Degree of
swelling: 0.5 mm or less) Ex. 2-20 0.50 0.13 1:2 2.0 67 46 No burst
No burst PE Alumina (79.degree. C.) (Degree of swelling: 0.5 mm or
less) Ex. 2-21 0.10 0.10 1:2 2.0 76 44 No burst No burst PE Alumina
(69.degree. C.) (Degree of swelling: 0.5 mm or less) Ex. 2-22 4.0
4.0 1:2 4.0 82 43 No burst No burst PE Alumina (63.degree. C.)
(Degree of swelling: 0.5 mm or less)
TABLE-US-00004 TABLE 4 Average Average Overcharge size [.mu.m] size
[.mu.m] Content Thickness test Temperature and and ratio of Rate 3
C-10 V raising test material for material for (organic protecting
characteristics (state of cell 25~150.degree. C. organic inorganic
particle:inorganic layer Impedance 5 C/1 C & cell (state of
cell & particle particle particle) [.mu.m] [m.OMEGA.] [%]
temperature) swelling) Com. Ex. 2-1 -- -- -- -- 70 51 Firing Smoke
generation Com. Ex. 2-2 -- 0.20 -- 2.0 76 45 Burst No burst Alumina
(Degree of swelling: 0.7 mm) Com. Ex. 2-3 0.50 -- -- 2.0 75 47 No
burst Smoke PE (63.degree. C.) generation Com. Ex. 2-4 0.50 0.20
2:1 2.0 74 49 No burst Burst PE Alumina (59.degree. C.) Com. Ex.
2-5 0.50 0.20 1:5 2.0 75 46 Burst No burst PE Alumina (Degree of
swelling: 0.8 mm) Com. Ex. 2-6 0.05 0.05 1:2 2.0 92 38 No burst No
burst PE Alumina (72.degree. C.) (Degree of swelling: 0.8 mm) Com.
Ex. 2-7 6.0 6.0 1:2 7.0 116 20 No burst No burst PE Alumina
(58.degree. C.) (Degree of swelling: 0.8 mm) Com. Ex. 2-8 0.50 0.2
1:2 2.0 75 46 Burst No burst PTFE Alumina (Degree of swelling: 0.8
mm)
Example 3-1
[0281] (Preparation of Negative Electrode)
[0282] First, 1.5 parts by mass of sodium carboxymethylcellulose
(trade name: Cellogen WS-C manufactured by Dai-ichi Kogyo Seiyaku
Co., Ltd.) was dissolved in pure water (purified through an ion
exchange membrane and distillated). To the dissolution solution,
93.5 parts by mass of natural graphite (trade name: HG-702,
manufactured by Hitachi Chemical Co., Ltd.), 2.0 parts by mass of
acetylene black (trade name: Denka Black, manufactured by Denki
Kagaku Kogyo K.K.) and 3.0 parts by mass of styrene-butadiene
rubber (trade name: SN-307R, manufactured by Nippon A&L Inc.)
were added and mixed and dispersed by a planetary mixer to obtain a
slurry-state coating solution for forming a negative electrode
active-material layer. The coating solution was applied to both
surfaces of a copper foil having a thickness of 15 .mu.m by a
doctor blade method, dried and pressed by a calender roll to form
in a negative electrode active-material layer having a thickness
(one surface) of 80 .mu.m.
[0283] Next, as a low-melting point organic particle, 31.8 parts by
mass of a polyethylene particle having a melting temperature of
130.degree. C. and an average particle size (D50) of 2.0 .mu.m and
obtained by classification from Flow beads HE series (trade name,
manufactured by Sumitomo Seika Chemicals Co., Ltd.); as a
high-melting point organic particle, 63.7 parts by mass of a
polytetrafluoroethylene particle (trade name: SST series, melting
temperature: 327.degree. C., an average particle size (D50): 2.0
.mu.m, manufactured by SHAMROCK TECHNOLOGIES), 3.0 parts by mass of
styrene-butadiene rubber (trade name: SN-307R manufactured by
Nippon A&L Inc.) and 1.5 parts by mass of sodium
carboxymethylcellulose (trade name: Cellogen WS-C manufactured by
Dai-ichi Kogyo Seiyaku Co., Ltd.) were mixed and dissolved in pure
water (purified through an ion exchange membrane and distillated)
to obtain a slurry-state coating solution for forming a protecting
layer. The coating solution was applied to each negative electrode
active-material layer by a doctor blade method and dried to obtain
a protecting layer having a thickness (one surface) of 3.0 .mu.m.
In this manner, a negative electrode (two-surface coated negative
electrode) having a negative electrode active-material layer and a
protecting layer formed on both surfaces of a collector was
obtained.
[0284] (Preparation of Positive Electrode)
[0285] First, 44.5 parts by mass of lithium nickel-cobalt manganate
(trade name: NCM-01ST-5, manufactured by Toda Kogyo Corp.), 44.5
parts by mass of lithium-manganese spinel (trade name: IPM-6050
manufactured by Toda Kogyo Corp.), 3.0 parts by mass of acetylene
black (trade name: Denka Black, manufactured by Denki Kagaku Kogyo
K.K.), 3.0 parts by mass of graphite (trade name: KS-6,
manufactured by Timcal Ltd.) and 5.0 parts by mass of polyvinyldene
fluoride (PVDF) (trade name: KYNAR-761, manufactured by Arkema
Inc.) were mixed and dissolved in N-methylpyrrolidone (NMP) to
obtain a slurry-state coating solution for forming a positive
electrode active-material layer. The coating solution was applied
to both surfaces of an aluminum foil having a thickness of 20 .mu.m
by a doctor blade method, dried and pressed by a calender roll to
form a positive electrode active-material layer having a thickness
(one surface) of 95 .mu.m. In this manner, a positive electrode
(two-surface coated positive electrode) having a positive electrode
active-material layer formed on both surfaces of a collector was
obtained.
[0286] (Preparation of Electrolytic Solution)
[0287] First, 20 parts by volume of propylene carbonate (PC), 10
parts by volume of ethylene carbonate (EC) and 70 parts by volume
of diethyl carbonate were mixed to obtain a solvent mixture. In the
solvent mixture, lithium hexafluorophosphate (LiPF.sub.6) was
dissolved so as to obtain a concentration of 1.5 moldm.sup.-3 to
obtain an electrolytic solution.
[0288] (Preparation of Lithium Ion Secondary Battery)
[0289] A two-surface coated negative electrode (dimension of 31.0
mm.times.41.5 mm) in a shape having a tongue portion was obtained
by stamping. A two-surface coated positive electrode (dimension of
30.5 mm.times.41.0 mm) in a shape having a tongue portion was
obtained by stamping. Furthermore, a separator formed of
polyethylene and having a dimension of 32.0 mm.times.43.0 mm was
prepared. Six sheets of two-surface coated negative electrodes and
five sheets of two-surface coated positive electrodes were
alternately laminated with the separator interposed between them to
form a laminate having the laminate structure: two-surface coated
negative electrode/separator/two-surface coated positive
electrode/separator/two-surface coated negative
electrode/separator/two-surface coated positive
electrode/separator/two-surface coated negative
electrode/separator/two-surface coated positive
electrode/separator/two-surface coated negative
electrode/separator/two-surface coated positive
electrode/separator/two-surface coated negative
electrode/separator/two-surface coated positive
electrode/separator/two-surface coated negative electrode. The
resultant laminate structure was housed in an aluminum laminate
film and an electrolytic solution was injected and sealed under
vacuum. In this manner, a lithium ion secondary battery was
prepared, which had the same structure as that shown in FIG. 5 and
FIG. 6 except the number of two-surface coated negative electrodes
and the number of two-surface coated positive electrodes
laminated.
Example 3-2
[0290] A lithium ion secondary battery was prepared in the same
manner as in Example 3-1 except that, as the low-melting point
organic particle of the protecting layer, use was made of a
polyethylene particle having a melting temperature of 105.degree.
C. and an average particle size (D50) of 2.0 .mu.m and obtained by
classification from Flow beads LE series, (trade name, manufactured
by Sumitomo Seika Chemicals Co., Ltd).
Example 3-3
[0291] A lithium ion secondary battery was prepared in the same
manner as in Example 3-1 except that, as the low-melting point
organic particle of the protecting layer, use was made of a
polypropylene particle having a melting temperature of 170.degree.
C., an average particle size (D50) of 2.0 .mu.m and obtained by
classification from polypropylene particle [TRL-PP-101series]
(trade name, manufactured by Trial Corporation).
Example 3-4
[0292] A lithium ion secondary battery was prepared in the same
manner as in Example 3-1 except that, as the high-melting point
organic particle of the protecting layer, use was made of a
poly(methyl methacrylate) particle having a melting temperature of
195.degree. C. and an average particle size (D50) of 2.0 .mu.m and
obtained by classification from High pearl series (acrylic beads)
(trade name, manufactured by Negami Chemical Industrial Co.,
Ltd).
Example 3-5
[0293] A lithium ion secondary battery was prepared in the same
manner as in Example 3-1 except that, as the high-melting point
organic particle of the protecting layer, use was made of a
polyimide particle (synthesized by an isocyanate method, melting
temperature: 300.degree. C. or more (not softened at a temperature
less than 300.degree. C.), an average particle size (D50): 2.0
.mu.m).
[0294] Note that, the polyimide particle was prepared by an
isocyanate method as follows. First, 0.1 mole of
3,3',4,4'-benzophenone tetracarboxylic acid dianhydride (BTDA) was
dissolved in 224 g of N-methyl-2-pyrrolidone (NMP) to prepare a
solution. While heating and stirring at 140.degree. C., 0.05 mole
of triethylenediamine (TEDA) serving as a catalyst was added to the
solution and dispersed well. Subsequently, 0.1 mole of
2,4-tolylenediisocyanate (TDI) was added, mixed and stirred for 24
hours to precipitate a microparticle of a polyimide precursor.
Thereafter, the polyimide-precursor microparticle was recovered by
a centrifugal machine and washed with acetone. The centrifugation
and washing were repeatedly performed to purify the
polyimide-precursor microparticle. Thereafter, the microparticle
was dispersed in N-methyl-2-pyrrolidone (NMP) and refluxed at
190.degree. C. for 5 hours and a polymerization reaction was
continued. After completion of the reaction, the reaction was
cooled and filtrated to obtain polyimide, which was washed with
acetone, dried to obtain a polyimide particle. The resultant
polyimide particle was classified to obtain a particle having an
average particle size (D50) of 2.0 .mu.m. This was used as the
high-melting point organic particle of the example.
Example 3-6
[0295] A lithium ion secondary battery was prepared in the same
manner as in Example 34 except that, in the protecting layer, the
content of the low-melting point organic particle was set to 47.7
parts by mass and the content of the high-melting point organic
particle was set to 47.8 parts by mass.
Example 3-7
[0296] A lithium ion secondary battery was prepared in the same
manner as in Example 3-1 except that, in the protecting layer, the
content of the low-melting point organic particle was set to 53.0
parts by mass and the content of the high-melting point organic
particle was set to 42.5 parts by mass.
Example 3-8
[0297] A lithium ion secondary battery was prepared in the same
manner as in Example 3-1 except that, in the protecting layer, the
content of the low-melting point organic particle was set to 19.1
parts by mass and the content of the high-melting point organic
particle was set to 76.4 parts by mass.
Example 3-9
[0298] A lithium ion secondary battery was prepared in the same
manner as in Example 3-1 except that, in the protecting layer, the
content of the low-melting point organic particle was set to 17.4
parts by mass and the content of the high-melting point organic
particle was set to 78.1 parts by mass.
Comparative Example 3-1
[0299] A lithium ion secondary battery was prepared in the same
manner as in Example 3-1 except that, in the protecting layer, as
the low-melting point organic particle, use was made of an
ethylene-vinyl acetate copolymer (EVA) particle having a melting
temperature of 80.degree. C., an average particle size (D50) of 2.0
.mu.m (an ethylene-vinyl acetate copolymer particle extracted from
Aquatech EC-1700 (trade name) solution and classified, manufactured
by CHIRIKA. Co., Ltd).
Comparative Example 3-2
[0300] A lithium ion secondary battery was prepared in the same
manner as in Example 3-1 except that, in the protecting layer, as
the low-melting point organic particle, use was made of a
benzoguanamine (BG) particle having a melting temperature of
228.degree. C. and an average particle size (D50) of 2.0 .mu.m, and
obtained by classification from EPOSTAR (trade name, registered
trade mark), MS grade, manufactured by NIPPON SHOKUBAI CO.,
LTD).
Comparative Example 3-3
[0301] A lithium ion secondary battery was prepared in the same
manner as in Example 3-1 except that, in the protecting layer, as
the high-melting point organic particle, use was made of a
polyphenylene sulfide (PPS) particle having a melting temperature
of 282.degree. C. and an average particle size (D50) of 2.0 .mu.m
(trade name: FORTRON, 0220A9 grade]), which was pulverized and
classified to put in use, manufactured by Polyplastics Co.,
Ltd).
Comparative Example 3-4
[0302] A lithium ion secondary battery was prepared in the same
manner as in Example 3-1 except that, in the protecting layer, the
content of the low-melting point organic particle was set to 95.5
parts by mass, and the high-melting point organic particle was not
added.
Comparative Example 3-5
[0303] A lithium ion secondary battery was prepared in the same
manner as in Example 3-1 except that, in the protecting layer, the
low-melting point organic particle was not added, and the content
of the high-melting point organic particle was 95.5 parts by
mass.
Comparative Example 3-6
[0304] A lithium ion secondary battery was prepared in the same
manner as in Example 3-1 except that, in the protecting layer, the
low-melting point organic particle and the high-melting point
organic particle were not added, and 95.5 parts by mass of an
alumina particle (trade name: AL series, an average particle size
(D50): 2.0 .mu.m manufactured by Sumitomo Chemical Co., Ltd.) was
added as the inorganic particle.
Comparative Example 3-7
[0305] A lithium ion secondary battery was prepared in the same
manner as in Example 3-1 except that, in the protecting layer, the
low-melting point organic particle was not added; 31.8 parts by
mass of an alumina particle (trade name: AL series, an average
particle size (D50): 2.0 .mu.m, manufactured by Sumitomo Chemical
Co., Ltd.) was added as the inorganic particle; and the content of
the high-melting point organic particle was set to 63.7 parts by
mass.
Comparative Example 3-8
[0306] A lithium ion secondary battery was prepared in the same
manner as in Example 3-1 except that the protecting layer was not
provided.
[0307] <Measurement of Rate Characteristics>
[0308] With respect to the lithium ion secondary batteries obtained
in Examples and Comparative Examples, discharged capacity was
measured at 1 C (the amount of current discharged for one hour when
constant-current discharge is performed at 25.degree. C.) and at 5
C (the amount of current discharged for 0.2 hours when
constant-current discharge is performed at 25.degree. C.). The
ratio (%) of discharged capacity obtained at 5 C to the discharged
capacity (regarded as 100%) at 1 C was obtained as rate
characteristics. The results are shown in Tables 5 and 6.
[0309] <Measurement of Impedance Increase Rate>
[0310] The lithium ion secondary batteries obtained in Examples and
Comparative Examples were each measured for impedance (m.OMEGA.) at
an alternate current of 1 kHz by an impedance analyzer (SI 1287, SI
1260) manufactured by Toyo Corporation. This was regarded as an
initial impedance. Next, the lithium ion secondary battery was
previously subjected to a charge-discharge operation performed at a
rate as low as 0.05 C at 25.degree. C. and a CC charge operation
and a CV charge operation were performed under an environment
temperature of 50.degree. C. at 1 C until 4.2 V, and thereafter,
discharged at 1 C up to 3.0 V. The charge-discharge operation
(regarded it as a single cycle) was repeated up to 100 times.
Thereafter, the lithium ion secondary battery was measured for
impedance (m.OMEGA.) at an alternate current of 1 kHz by an
impedance analyzer (SI 1287, SI 1260) manufactured by Toyo
Corporation, which valve was used as the impedance after the cycle
test. Note that impedance was measured at an environment
temperature of 25.degree. C. and a relative humidity of 60%. An
impedance increase rate was obtained in accordance with the
following expression (A), wherein the initial impedance was
represented by R1 and the impedance after the cycle test was
represented by R2. The results are shown in Tables 5 and 6.
Impedance increase rate (%)={(R2-R1)/R1}.times.100 (A)
[0311] <Overcharge Test>
[0312] The lithium ion secondary batteries obtained in Examples and
Comparative Examples were each previously subjected to a
charge-discharge operation at a rate as low as 0.05 C at 25.degree.
C. and then subjected to a CC charge operation performed at 3 C
until 10 V, and thereafter, CV charge was maintained until the
temperature of the cell decreased. The maximum surface temperature
of the cell surface was `obtained based on temperature measurement
and states of cells were observed. These measurement results are
shown in Tables 5 and 6. Note that, in the overcharge test, the
lithium ion secondary batteries that caused no cell burst can be
evaluated that the protecting layer has a sufficient shutdown
function and safety during a heat-up time is excellent.
Furthermore, in the batteries that caused no burst, the lower the
cell temperature, the more an increase of inner temperature of the
cell was suppressed, meaning that the battery is high in safety.
The cell temperature is preferably 80.degree. C. or less
[0313] <Temperature Raising Test>
[0314] The lithium ion secondary batteries obtained in Examples and
Comparative Examples were each previously subjected to a
charge-discharge process performed at a rate as low as 0.05 C at
25.degree. C. and then, subjected to a charge operation performed
at 1 C until 4.2 V. Thereafter, the thickness of the cell (before
storage) was measured. After that, the lithium ion secondary
battery was loaded in an oven and the temperature was increased at
a temperature raising rate of 5.degree. C./minute until the
temperature reached 150.degree. C. and stored at 150.degree. C. for
one hour. Thereafter, the state of the cell was observed. The cell
that did not burst was measured for thickness (after storage). The
difference in thickness before and after storage was obtained as a
degree of swelling (mm). The results are shown in Tables 5 and 6.
Note that, lithium ion secondary batteries that did not burst in
the temperature raising test can be evaluated that shrinkage of the
protecting layer during a heat-up time is sufficiently suppressed
and safety during a heat-up time is excellent.
TABLE-US-00005 TABLE 5 Material Material and and melting melting
Content point of point of ratio low- high- (low- Overcharge
Temperature melting melting Material melting Rate Impedance test 3
C-10 V raising test 25 point point for point:high- characteristics
increase (state of cell to 150.degree. C. organic organic inorganic
melting 5 C/1 C rate & cell (state of cell particle particle
particle point) [%] [%] temperature) & swelling) Ex. 3-1 PE
PTFE -- 1:2 48 10 No burst No burst (130.degree. C.) (327.degree.
C.) (69.degree. C.) (Degree of swelling: less than 0.5 mm) Ex. 3-2
PE PTFE -- 1:2 48 18 No burst No burst (105.degree. C.)
(327.degree. C.) (69.degree. C.) (Degree of swelling: less than 0.5
mm) Ex. 3-3 PP PTFE -- 1:2 47 9 No burst No burst (170.degree. C.)
(327.degree. C.) (72.degree. C.) (Degree of swelling: less than 0.5
mm) Ex. 3-4 PMMA PTFE -- 1:2 47 9 No burst No burst (195.degree.
C.) (327.degree. C.) (75.degree. C.) (Degree of swelling: less than
0.5 mm) Ex. 3-5 PE PI -- 1:2 48 10 No burst No burst (130.degree.
C.) (.gtoreq.300.degree. C.) (68.degree. C.) (Degree of swelling:
less than 0.5 mm) Ex. 3-6 PE PTFE -- 1:1 48 12 No burst No burst
(130.degree. C.) (327.degree. C.) (67.degree. C.) (Degree of
swelling: 0.85 mm) Ex. 3-7 PE PTFE -- 1:0.8 48 13 No burst No burst
(130.degree. C.) (327.degree. C.) (70.degree. C.) (Degree of
swelling: 1.7 mm) Ex. 3-8 PE PTFE -- 1:4 48 10 No burst No burst
(130.degree. C.) (327.degree. C.) (76.degree. C.) (Degree of
swelling: 0.65 mm) Ex. 3-9 PE PTFE -- 1:4.5 48 9 No burst No burst
(130.degree. C.) (327.degree. C.) (83.degree. C.) (Degree of
swelling: 0.8 mm)
TABLE-US-00006 TABLE 6 Material Material and and melting melting
Content point of point of ratio low- high- (low- Overcharge
Temperature melting melting Material melting Rate Impedance test 3
C-10 V raising test 25 point point for point:high- characteristics
increase (state of cell to 150.degree. C. organic organic inorganic
melting 5 C/1 C rate & cell (state of cell particle particle
particle point) [%] [%] temperature) & swelling) Com. Ex. EVA
PTFE -- 1:2 47 76 No burst No burst 3-1 (80.degree. C.)
(327.degree. C.) (68.degree. C.) (Degree of swelling: 0.8 mm) Com.
Ex. BG PTFE -- 1:2 48 11 Burst No burst 3-2 (228.degree. C.)
(327.degree. C.) (Degree of swelling: 0.9 mm) Com. Ex. PE PPS --
1:2 48 10 No burst Burst 3-3 (130.degree. C.) (282.degree. C.)
(71.degree. C.) Com. Ex. PE -- -- -- 47 11 No burst Smoke 3-4
(130.degree. C.) (72.degree. C.) generation Com. Ex. -- PTFE -- --
49 12 Burst No burst 3-5 (327.degree. C.) (Degree of swelling: 0.9
mm) Com. Ex. -- -- Alumina -- 39 11 Burst No burst 3-6 (Degree of
swelling: 1.2 mm) Com. Ex. -- PTFE Alumina 1:2 46 11 Burst No burst
3-7 (327.degree. C.) (Degree of swelling: 1.0 mm) Com. Ex. -- -- --
-- 51 10 Firing Smoke 3-8 generation
Example 4-1
[0315] First, 90.0 parts by mass of a natural graphite particle
(trade name "HG-706" manufactured by Hitachi Chemical Co., Ltd.), 2
parts by mass of acetylene black serving as a conductive auxiliary
and 8 parts by mass of PVDF serving as a binder were mixed and
dispersed by a planetary mixer. After the viscosity was controlled
with N-methyl-2-pyrrolidone, the solution was mixed and dispersed
by Gaulin homogenizer to prepare a slurry. The slurry was applied
onto one of the surfaces of a copper foil serving as a negative
electrode collector and having a thickness of 15 .mu.m and dried.
Similarly, the slurry was applied onto the other surface of the
copper foil, dried and pressed by a roll to form a negative
electrode active-material layer having a thickness of 85 .mu.m on
both surfaces of the copper foil.
[0316] Thereafter, 92.0 parts by mass of a silicone resin particle
(trade name: "TOSPEARL" manufactured by MOMENTIVE PERFORMANCE
MATERIALS, an average particle size (D50): 2.0 .mu.m, aspect ratio:
1.03) and PVDF (8 parts by mass) serving as a binder were mixed and
dispersed by a planetary mixer. After the viscosity was controlled
with N-methyl-2-pyrrolidone to prepare a slurry. The slurry was
applied onto the negative electrode active-material layer formed on
each of the surfaces of a copper foil and dried to form a negative
electrode protecting layer having a thickness of 2.0 .mu.m. In this
manner, a negative electrode (two-surface coated negative
electrode) having a negative electrode active-material layer and a
negative electrode protecting layer formed on both surfaces of a
collector was obtained. Note that, an average particle size (D50)
was calculated based on the measurement data obtained by a
Microtrack HRA (trade name, manufactured by Microtrack Co., Ltd.).
The aspect ratio was obtained by calculation as an average of
major-axis length/minor-axis length values of arbitrarily chosen 10
silicone resin particles under an electron microscope.
Examples 4-2 to 4-7
[0317] A negative electrode was prepared in the same manner as in
5. Example 4-1 except that use was made of the silicone resin
particles having D50 and an aspect ratio shown in FIG. 7
manufactured by MOMENTIVE PERFORMANCE MATERIALS (more specifically,
Example 4-2 employed a silicone resin particle obtained by
classifying from TOSPEARL (trade name) and Examples 4-3 to 4-7
employed a silicone resin particle obtained by classifying from
"XC99-A8808" (trade name).
Comparative Example 4-1
[0318] A negative electrode was prepared in the same manner as in
Example 4-1 except that a negative electrode protecting layer
having a thickness of 2.0 .mu.m was formed of a polyethylene (PE)
particle having D50: 2.0 .mu.m and an aspect ratio: 1.03 and
obtained by classifying from Flow beads LE series (trade name,
manufactured by Sumitomo Seika Chemicals Co., Ltd.) in place of the
silicone resin particle.
Comparative Example 4-2
[0319] A negative electrode was prepared in the same manner as in
Example 4-1 except that a negative electrode protecting layer
having a thickness of 2.0 .mu.m was formed of a PTFE particle
(trade name "SST series" manufactured by SHAMROCK) having D50 of
2.0 .mu.m and an aspect ratio of 1.03 in place of the silicone
resin particle.
Comparative Example 4-3
[0320] A negative electrode was prepared in the same manner as in
Example 4-1 except that a negative electrode protecting layer
having a thickness of 2.0 to 4.0 .mu.m was formed of an alumina
particle (trade name: "AL series" manufactured by Sumitomo Chemical
Co., Ltd.) having D50 of 2.0 .mu.m and an aspect ratio of 1.86 in
place of the silicone resin particle.
[0321] A positive electrode was prepared by forming a positive
electrode active-material layer containing a positive electrode
active-material particle (LiCo.sub.1/3Ni.sub.1/3Mn.sub.1/3O.sub.2:
44.5 parts by mass, and LiMn.sub.2O.sub.4: 44.5 parts by mass), a
binder (PVDF: 5.0 parts by mass) and a conductive auxiliary
(acetylene black: 3.0 parts by mass, and graphite: 3.0 parts by
mass) on an aluminum collector. A lithium ion secondary battery was
prepared using porous polyethylene as a separator, a 1M
LiPF.sub.6-containing PC/EC/DEC (the ratio (parts by volume) of
20:10:70) as an electrolytic solution and each of the electrodes
obtained in the aforementioned Examples and Comparative Examples as
a negative electrode.
[0322] <Rate Characteristics>
[0323] With respect to the lithium ion secondary batteries obtained
above, discharged capacity was measured at 1 C (the amount of
current discharged for one hour when constant-current discharge is
performed at 25.degree. C.) and at 5 C (the amount of current
discharged for 0.2 hours when constant-current discharge is
performed at 25.degree. C.) and rate characteristics (discharged
capacity at 5 C/discharged capacity 1 C) was obtained. The results
are shown in Table 7.
[0324] <Overcharge Rest>
[0325] Constant current charge was performed at 2 C. After the
voltage reached 10 V, constant voltage charge was performed for 45
minutes. After the test, the state of the cell was visually
observed (with respect to the presence or absence of cell burst due
to melting or thermal decomposition). The results are shown in
Table 7.
TABLE-US-00007 TABLE 7 Thickness of Rate Material for protecting
characteristics protecting-layer D50 layer (5 C/1 C) State of cell
after particle [.mu.m] Aspect ratio [.mu.m] [%] overcharge test Ex.
4-1 Silicone resin 2.0 1.03 2.0 58 Not burst Ex. 4-2 0.5 1.03 2.0
54 Not burst Ex. 4-3 4.0 1.03 4.0 54 Not burst Ex. 4-4 2.0 1.30 2.0
55 Not burst Ex. 4-5 2.0 1.50 2.0 48 Not burst Ex. 4-6 0.3 1.03 2.0
50 Not burst Ex. 4-7 6.0 1.03 6.0 48 Not burst Com. Ex. 4-1 PE 2.0
1.03 2.0 58 Firing Com. Ex. 4-2 PTFE 2.0 1.03 2.0 55 Burst Com. Ex.
4-3 Alumina 2.0 1.86 2.0~4.0 45 Not burst
* * * * *